diff --git a/src/Adapter/AdapterInterface.php b/src/Adapter/AdapterInterface.php
index 2da88fe..fdb0a21 100644
--- a/src/Adapter/AdapterInterface.php
+++ b/src/Adapter/AdapterInterface.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Adapter;
 
 use Closure;
@@ -151,20 +160,20 @@ interface AdapterInterface
      *
      * @param $model
      * @param HasOne $relationship
-     * @param array $fields
+     * @param bool $linkage
      * @return mixed|null
      */
-    public function getHasOne($model, HasOne $relationship, array $fields = null);
+    public function getHasOne($model, HasOne $relationship, bool $linkage);
 
     /**
      * Get a list of models for a has-many relationship for the model.
      *
      * @param $model
      * @param HasMany $relationship
-     * @param array $fields
+     * @param bool $linkage
      * @return array
      */
-    public function getHasMany($model, HasMany $relationship, array $fields = null): array;
+    public function getHasMany($model, HasMany $relationship, bool $linkage): array;
 
     /**
      * Apply an attribute value to the model.
@@ -219,9 +228,11 @@ interface AdapterInterface
      * @param array $relationships
      * @param Closure $scope Should be called to give the deepest relationship
      *   an opportunity to scope the query that will fetch related resources
+     * @param bool $linkage true if we just need the IDs of the related
+     *   resources and not their full data
      * @return mixed
      */
-    public function load(array $models, array $relationships, Closure $scope): void;
+    public function load(array $models, array $relationships, Closure $scope, bool $linkage): void;
 
     /**
      * Load information about the IDs of related resources onto a collection
@@ -231,5 +242,5 @@ interface AdapterInterface
      * @param Relationship $relationship
      * @return mixed
      */
-    public function loadIds(array $models, Relationship $relationship): void;
+    // public function loadIds(array $models, Relationship $relationship): void;
 }
diff --git a/src/Adapter/EloquentAdapter.php b/src/Adapter/EloquentAdapter.php
index ddf12eb..b327b1b 100644
--- a/src/Adapter/EloquentAdapter.php
+++ b/src/Adapter/EloquentAdapter.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Adapter;
 
 use Closure;
@@ -69,12 +78,15 @@ class EloquentAdapter implements AdapterInterface
         return $model->{$this->getAttributeProperty($attribute)};
     }
 
-    public function getHasOne($model, HasOne $relationship, array $fields = null)
+    public function getHasOne($model, HasOne $relationship, bool $linkage)
     {
         $relation = $this->getEloquentRelation($model, $relationship);
 
-        // comment
-        if ($fields === ['id'] && $relation instanceof BelongsTo) {
+        // If it's a belongs-to relationship and we only need to get the ID,
+        // then we don't have to actually load the relation because the ID is
+        // stored in a column directly on the model. We will mock up a related
+        // model with the value of the ID filled.
+        if ($linkage && $relation instanceof BelongsTo) {
             if ($key = $model->{$relation->getForeignKeyName()}) {
                 $related = $relation->getRelated();
 
@@ -87,7 +99,7 @@ class EloquentAdapter implements AdapterInterface
         return $this->getRelationValue($model, $relationship);
     }
 
-    public function getHasMany($model, HasMany $relationship, array $fields = null): array
+    public function getHasMany($model, HasMany $relationship, bool $linkage): array
     {
         $collection = $this->getRelationValue($model, $relationship);
 
@@ -188,38 +200,42 @@ class EloquentAdapter implements AdapterInterface
         $query->take($limit)->skip($offset);
     }
 
-    public function load(array $models, array $relationships, Closure $scope): void
+    public function load(array $models, array $relationships, Closure $scope, bool $linkage): void
     {
+        // TODO: Find the relation on the model that we're after. If it's a
+        // belongs-to relation, and we only need linkage, then we won't need
+        // to load anything as the related ID is store directly on the model.
+
         (new Collection($models))->loadMissing([
             $this->getRelationshipPath($relationships) => $scope
         ]);
     }
 
-    public function loadIds(array $models, Relationship $relationship): void
-    {
-        if (empty($models)) {
-            return;
-        }
-
-        $property = $this->getRelationshipProperty($relationship);
-        $relation = $models[0]->$property();
-
-        // If it's a belongs-to relationship, then the ID is stored on the model
-        // itself, so we don't need to load anything in advance.
-        if ($relation instanceof BelongsTo) {
-            return;
-        }
-
-        (new Collection($models))->loadMissing([
-            $property => function ($query) use ($relation) {
-                $query->select($relation->getRelated()->getKeyName());
-
-                if (! $relation instanceof BelongsToMany) {
-                    $query->addSelect($relation->getForeignKeyName());
-                }
-            }
-        ]);
-    }
+    // public function loadIds(array $models, Relationship $relationship): void
+    // {
+    //     if (empty($models)) {
+    //         return;
+    //     }
+    //
+    //     $property = $this->getRelationshipProperty($relationship);
+    //     $relation = $models[0]->$property();
+    //
+    //     // If it's a belongs-to relationship, then the ID is stored on the model
+    //     // itself, so we don't need to load anything in advance.
+    //     if ($relation instanceof BelongsTo) {
+    //         return;
+    //     }
+    //
+    //     (new Collection($models))->loadMissing([
+    //         $property => function ($query) use ($relation) {
+    //             $query->select($relation->getRelated()->getKeyName());
+    //
+    //             if (! $relation instanceof BelongsToMany) {
+    //                 $query->addSelect($relation->getForeignKeyName());
+    //             }
+    //         }
+    //     ]);
+    // }
 
     private function getAttributeProperty(Attribute $attribute): string
     {
diff --git a/src/ErrorProviderInterface.php b/src/ErrorProviderInterface.php
index 4b55237..83e8b4f 100644
--- a/src/ErrorProviderInterface.php
+++ b/src/ErrorProviderInterface.php
@@ -1,10 +1,29 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer;
 
+use JsonApiPhp\JsonApi\Error;
+
 interface ErrorProviderInterface
 {
+    /**
+     * Get JSON:API error objects that represent this error.
+     *
+     * @return Error[]
+     */
     public function getJsonApiErrors(): array;
 
+    /**
+     * Get the most generally applicable HTTP error code for this error.
+     */
     public function getJsonApiStatus(): string;
 }
diff --git a/src/Exception/BadRequestException.php b/src/Exception/BadRequestException.php
index 3523e07..c249cf5 100644
--- a/src/Exception/BadRequestException.php
+++ b/src/Exception/BadRequestException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use DomainException;
diff --git a/src/Exception/ForbiddenException.php b/src/Exception/ForbiddenException.php
index 2113190..c069f1c 100644
--- a/src/Exception/ForbiddenException.php
+++ b/src/Exception/ForbiddenException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use DomainException;
diff --git a/src/Exception/InternalServerErrorException.php b/src/Exception/InternalServerErrorException.php
index 3dc5fe9..99dc9a6 100644
--- a/src/Exception/InternalServerErrorException.php
+++ b/src/Exception/InternalServerErrorException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use JsonApiPhp\JsonApi\Error;
diff --git a/src/Exception/MethodNotAllowedException.php b/src/Exception/MethodNotAllowedException.php
index b6b63db..1698f24 100644
--- a/src/Exception/MethodNotAllowedException.php
+++ b/src/Exception/MethodNotAllowedException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use DomainException as DomainExceptionAlias;
diff --git a/src/Exception/NotAcceptableException.php b/src/Exception/NotAcceptableException.php
index 0b4b875..5dd9cc6 100644
--- a/src/Exception/NotAcceptableException.php
+++ b/src/Exception/NotAcceptableException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use JsonApiPhp\JsonApi\Error;
diff --git a/src/Exception/NotImplementedException.php b/src/Exception/NotImplementedException.php
index 0af1992..a9d275f 100644
--- a/src/Exception/NotImplementedException.php
+++ b/src/Exception/NotImplementedException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use DomainException;
diff --git a/src/Exception/ResourceNotFoundException.php b/src/Exception/ResourceNotFoundException.php
index eef6d49..a1335c9 100644
--- a/src/Exception/ResourceNotFoundException.php
+++ b/src/Exception/ResourceNotFoundException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use JsonApiPhp\JsonApi\Error;
diff --git a/src/Exception/UnauthorizedException.php b/src/Exception/UnauthorizedException.php
index a11ad89..7a28296 100644
--- a/src/Exception/UnauthorizedException.php
+++ b/src/Exception/UnauthorizedException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use DomainException;
diff --git a/src/Exception/UnprocessableEntityException.php b/src/Exception/UnprocessableEntityException.php
index 6a01632..8ffb385 100644
--- a/src/Exception/UnprocessableEntityException.php
+++ b/src/Exception/UnprocessableEntityException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use DomainException;
diff --git a/src/Exception/UnsupportedMediaTypeException.php b/src/Exception/UnsupportedMediaTypeException.php
index 46ffdf2..ce755c5 100644
--- a/src/Exception/UnsupportedMediaTypeException.php
+++ b/src/Exception/UnsupportedMediaTypeException.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Exception;
 
 use JsonApiPhp\JsonApi\Error;
diff --git a/src/Handler/Concerns/FindsResources.php b/src/Handler/Concerns/FindsResources.php
index a37e32a..3ec0d7e 100644
--- a/src/Handler/Concerns/FindsResources.php
+++ b/src/Handler/Concerns/FindsResources.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Handler\Concerns;
 
 use Psr\Http\Message\ServerRequestInterface as Request;
@@ -9,13 +18,18 @@ use function Tobyz\JsonApiServer\run_callbacks;
 
 trait FindsResources
 {
+    /**
+     * Find a resource within the API after applying scopes for the resource type.
+     *
+     * @throws ResourceNotFoundException if the resource is not found.
+     */
     private function findResource(Request $request, ResourceType $resource, string $id)
     {
         $adapter = $resource->getAdapter();
 
         $query = $adapter->query();
 
-        run_callbacks($resource->getSchema()->getScopes(), [$query, $request, $id]);
+        run_callbacks($resource->getSchema()->getListeners('scope'), [$query, $request, $id]);
 
         $model = $adapter->find($query, $id);
 
diff --git a/src/Handler/Concerns/IncludesData.php b/src/Handler/Concerns/IncludesData.php
index e5706ad..a516f81 100644
--- a/src/Handler/Concerns/IncludesData.php
+++ b/src/Handler/Concerns/IncludesData.php
@@ -1,15 +1,27 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Handler\Concerns;
 
-use Closure;
 use Psr\Http\Message\ServerRequestInterface as Request;
-use function Tobyz\JsonApiServer\evaluate;
 use Tobyz\JsonApiServer\Exception\BadRequestException;
+use Tobyz\JsonApiServer\JsonApi;
 use Tobyz\JsonApiServer\ResourceType;
 use Tobyz\JsonApiServer\Schema\Relationship;
 use function Tobyz\JsonApiServer\run_callbacks;
 
+/**
+ * @property JsonApi $api
+ * @property ResourceType $resource
+ */
 trait IncludesData
 {
     private function getInclude(Request $request): array
@@ -32,10 +44,9 @@ trait IncludesData
         $tree = [];
 
         foreach (explode(',', $include) as $path) {
-            $keys = explode('.', $path);
             $array = &$tree;
 
-            foreach ($keys as $key) {
+            foreach (explode('.', $path) as $key) {
                 if (! isset($array[$key])) {
                     $array[$key] = [];
                 }
@@ -52,7 +63,8 @@ trait IncludesData
         $fields = $resource->getSchema()->getFields();
 
         foreach ($include as $name => $nested) {
-            if (! isset($fields[$name])
+            if (
+                ! isset($fields[$name])
                 || ! $fields[$name] instanceof Relationship
                 || ! $fields[$name]->isIncludable()
             ) {
@@ -69,69 +81,42 @@ trait IncludesData
         }
     }
 
-    private function buildRelationshipTrails(ResourceType $resource, array $include): array
-    {
-        $fields = $resource->getSchema()->getFields();
-        $trails = [];
-
-        foreach ($include as $name => $nested) {
-            $relationship = $fields[$name];
-
-            if ($relationship->getLoadable()) {
-                $trails[] = [$relationship];
-            }
-
-            if ($type = $fields[$name]->getType()) {
-                $relatedResource = $this->api->getResource($type);
-
-                $trails = array_merge(
-                    $trails,
-                    array_map(
-                        function ($trail) use ($relationship) {
-                            return array_merge([$relationship], $trail);
-                        },
-                        $this->buildRelationshipTrails($relatedResource, $nested)
-                    )
-                );
-            }
-        }
-
-        return $trails;
-    }
-
     private function loadRelationships(array $models, array $include, Request $request)
     {
-        $adapter = $this->resource->getAdapter();
-        $fields = $this->resource->getSchema()->getFields();
+        $this->loadRelationshipsAtLevel($models, [], $this->resource, $include, $request);
+    }
 
-        $trails = $this->buildRelationshipTrails($this->resource, $include);
-        $loaded = [];
-
-        foreach ($trails as $relationships) {
-            $relationship = end($relationships);
-
-            if (($load = $relationship->getLoadable()) instanceof Closure) {
-                $load($models, $relationships, false, $request);
-            } else {
-                $scope = function ($query) use ($request, $relationship) {
-                    run_callbacks($relationship->getScopes(), [$query, $request]);
-                };
-
-                $adapter->load($models, $relationships, $scope);
-            }
-
-            $loaded[] = $relationships[0];
-        }
+    private function loadRelationshipsAtLevel(array $models, array $relationshipPath, ResourceType $resource, array $include, Request $request)
+    {
+        $adapter = $resource->getAdapter();
+        $fields = $resource->getSchema()->getFields();
 
         foreach ($fields as $name => $field) {
-            if (! $field instanceof Relationship || ! evaluate($field->getLinkage(), [$request]) || ! $field->getLoadable() || in_array($field, $loaded)) {
+            if (
+                ! $field instanceof Relationship
+                || (! $field->isLinkage() && ! isset($include[$name]))
+            ) {
                 continue;
             }
 
-            if (($load = $field->getLoadable()) instanceof Closure) {
-                $load($models, true);
-            } else {
-                $adapter->loadIds($models, $field);
+            $nextRelationshipPath = array_merge($relationshipPath, [$field]);
+
+            if ($load = $field->isLoadable()) {
+                if (is_callable($load)) {
+                    $load($models, $nextRelationshipPath, $field->isLinkage(), $request);
+                } else {
+                    $scope = function ($query) use ($request, $field) {
+                        run_callbacks($field->getListeners('scope'), [$query, $request]);
+                    };
+
+                    $adapter->load($models, $nextRelationshipPath, $scope, $field->isLinkage());
+                }
+            }
+
+            if (isset($include[$name]) && is_string($type = $field->getType())) {
+                $relatedResource = $this->api->getResource($type);
+
+                $this->loadRelationshipsAtLevel($models, $nextRelationshipPath, $relatedResource, $include[$name] ?? [], $request);
             }
         }
     }
diff --git a/src/Handler/Concerns/SavesData.php b/src/Handler/Concerns/SavesData.php
index 4548f21..1c17de0 100644
--- a/src/Handler/Concerns/SavesData.php
+++ b/src/Handler/Concerns/SavesData.php
@@ -1,34 +1,61 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Handler\Concerns;
 
 use Psr\Http\Message\ServerRequestInterface as Request;
-use function Tobyz\JsonApiServer\evaluate;
-use function Tobyz\JsonApiServer\get_value;
-use function Tobyz\JsonApiServer\has_value;
-use function Tobyz\JsonApiServer\set_value;
 use Tobyz\JsonApiServer\Exception\BadRequestException;
 use Tobyz\JsonApiServer\Exception\UnprocessableEntityException;
-use function Tobyz\JsonApiServer\run_callbacks;
+use Tobyz\JsonApiServer\JsonApi;
+use Tobyz\JsonApiServer\ResourceType;
 use Tobyz\JsonApiServer\Schema\Attribute;
 use Tobyz\JsonApiServer\Schema\HasMany;
 use Tobyz\JsonApiServer\Schema\HasOne;
 use Tobyz\JsonApiServer\Schema\Relationship;
+use function Tobyz\JsonApiServer\evaluate;
+use function Tobyz\JsonApiServer\get_value;
+use function Tobyz\JsonApiServer\has_value;
+use function Tobyz\JsonApiServer\run_callbacks;
+use function Tobyz\JsonApiServer\set_value;
 
+/**
+ * @property JsonApi $api
+ * @property ResourceType $resource
+ */
 trait SavesData
 {
     use FindsResources;
 
-    private function parseData($body): array
+    /**
+     * Parse and validate a JSON:API document's `data` member.
+     *
+     * @throws BadRequestException if the `data` member is invalid.
+     */
+    private function parseData($body, $model = null): array
     {
-        if (! is_array($body) && ! is_object($body)) {
-            throw new BadRequestException;
-        }
-
         $body = (array) $body;
 
-        if (! isset($body['data'])) {
-            throw new BadRequestException('Root data attribute missing');
+        if (! isset($body['data']) || ! is_array($body['data'])) {
+            throw new BadRequestException('data must be an object');
+        }
+
+        if (! isset($body['data']['type']) || $body['data']['type'] !== $this->resource->getType()) {
+            throw new BadRequestException('data.type does not match the resource type');
+        }
+
+        if ($model) {
+            $id = $this->resource->getAdapter()->getId($model);
+            if (! isset($body['data']['id']) || $body['data']['id'] !== $id) {
+                throw new BadRequestException('data.id does not match the resource ID');
+            }
         }
 
         if (isset($body['data']['attributes']) && ! is_array($body['data']['attributes'])) {
@@ -45,7 +72,12 @@ trait SavesData
         );
     }
 
-    private function getModelForIdentifier(Request $request, $identifier, array $validTypes = null)
+    /**
+     * Get the model corresponding to the given identifier.
+     *
+     * @throws BadRequestException if the identifier is invalid.
+     */
+    private function getModelForIdentifier(Request $request, array $identifier, array $validTypes = null)
     {
         if (! isset($identifier['type'])) {
             throw new BadRequestException('type not specified');
@@ -64,12 +96,20 @@ trait SavesData
         return $this->findResource($request, $resource, $identifier['id']);
     }
 
+    /**
+     * Assert that the fields contained within a data object are valid.
+     */
     private function validateFields(array $data, $model, Request $request)
     {
         $this->assertFieldsExist($data);
         $this->assertFieldsWritable($data, $model, $request);
     }
 
+    /**
+     * Assert that the fields contained within a data object exist in the schema.
+     *
+     * @throws BadRequestException if a field is unknown.
+     */
     private function assertFieldsExist(array $data)
     {
         $fields = $this->resource->getSchema()->getFields();
@@ -83,15 +123,23 @@ trait SavesData
         }
     }
 
+    /**
+     * Assert that the fields contained within a data object are writable.
+     *
+     * @throws BadRequestException if a field is not writable.
+     */
     private function assertFieldsWritable(array $data, $model, Request $request)
     {
         foreach ($this->resource->getSchema()->getFields() as $field) {
-            if (has_value($data, $field) && ! evaluate($field->getWritable(), [$model, $request])) {
+            if (has_value($data, $field) && ! evaluate($field->isWritable(), [$model, $request])) {
                 throw new BadRequestException("Field [{$field->getName()}] is not writable");
             }
         }
     }
 
+    /**
+     * Replace relationship linkage within a data object with models.
+     */
     private function loadRelatedResources(array &$data, Request $request)
     {
         foreach ($this->resource->getSchema()->getFields() as $field) {
@@ -102,7 +150,7 @@ trait SavesData
             $value = get_value($data, $field);
 
             if (isset($value['data'])) {
-                $allowedTypes = $field->getAllowedTypes();
+                $allowedTypes = (array) $field->getType();
 
                 if ($field instanceof HasOne) {
                     set_value($data, $field, $this->getModelForIdentifier($request, $value['data'], $allowedTypes));
@@ -117,12 +165,17 @@ trait SavesData
         }
     }
 
-    private function assertDataValid(array $data, $model, Request $request, bool $all): void
+    /**
+     * Assert that the field values within a data object pass validation.
+     *
+     * @throws UnprocessableEntityException if any fields do not pass validation.
+     */
+    private function assertDataValid(array $data, $model, Request $request, bool $validateAll): void
     {
         $failures = [];
 
         foreach ($this->resource->getSchema()->getFields() as $field) {
-            if (! $all && ! has_value($data, $field)) {
+            if (! $validateAll && ! has_value($data, $field)) {
                 continue;
             }
 
@@ -141,6 +194,9 @@ trait SavesData
         }
     }
 
+    /**
+     * Set field values from a data object to the model instance.
+     */
     private function setValues(array $data, $model, Request $request)
     {
         $adapter = $this->resource->getAdapter();
@@ -169,21 +225,30 @@ trait SavesData
         }
     }
 
+    /**
+     * Save the model and its fields.
+     */
     private function save(array $data, $model, Request $request)
     {
         $this->saveModel($model, $request);
         $this->saveFields($data, $model, $request);
     }
 
+    /**
+     * Save the model.
+     */
     private function saveModel($model, Request $request)
     {
-        if ($saver = $this->resource->getSchema()->getSaver()) {
-            $saver($model, $request);
+        if ($saveCallback = $this->resource->getSchema()->getSaveCallback()) {
+            $saveCallback($model, $request);
         } else {
             $this->resource->getAdapter()->save($model);
         }
     }
 
+    /**
+     * Save any fields that were not saved with the model.
+     */
     private function saveFields(array $data, $model, Request $request)
     {
         $adapter = $this->resource->getAdapter();
@@ -195,8 +260,8 @@ trait SavesData
 
             $value = get_value($data, $field);
 
-            if ($saver = $field->getSaver()) {
-                $saver($model, $value, $request);
+            if ($saveCallback = $field->getSaveCallback()) {
+                $saveCallback($model, $value, $request);
             } elseif ($field instanceof HasMany) {
                 $adapter->saveHasMany($model, $field, $value);
             }
@@ -205,6 +270,9 @@ trait SavesData
         $this->runSavedCallbacks($data, $model, $request);
     }
 
+    /**
+     * Run field saved listeners.
+     */
     private function runSavedCallbacks(array $data, $model, Request $request)
     {
         foreach ($this->resource->getSchema()->getFields() as $field) {
diff --git a/src/Handler/Create.php b/src/Handler/Create.php
index de99860..bd5ff90 100644
--- a/src/Handler/Create.php
+++ b/src/Handler/Create.php
@@ -1,15 +1,24 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Handler;
 
 use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
 use Psr\Http\Server\RequestHandlerInterface;
+use Tobyz\JsonApiServer\Exception\ForbiddenException;
+use Tobyz\JsonApiServer\JsonApi;
+use Tobyz\JsonApiServer\ResourceType;
 use function Tobyz\JsonApiServer\evaluate;
 use function Tobyz\JsonApiServer\has_value;
-use Tobyz\JsonApiServer\JsonApi;
-use Tobyz\JsonApiServer\Exception\ForbiddenException;
-use Tobyz\JsonApiServer\ResourceType;
 use function Tobyz\JsonApiServer\run_callbacks;
 use function Tobyz\JsonApiServer\set_value;
 
@@ -26,11 +35,16 @@ class Create implements RequestHandlerInterface
         $this->resource = $resource;
     }
 
+    /**
+     * Handle a request to create a resource.
+     *
+     * @throws ForbiddenException if the resource is not creatable.
+     */
     public function handle(Request $request): Response
     {
         $schema = $this->resource->getSchema();
 
-        if (! evaluate($schema->getCreatable(), [$request])) {
+        if (! evaluate($schema->isCreatable(), [$request])) {
             throw new ForbiddenException;
         }
 
@@ -56,9 +70,9 @@ class Create implements RequestHandlerInterface
 
     private function createModel(Request $request)
     {
-        $creator = $this->resource->getSchema()->getCreator();
+        $createModel = $this->resource->getSchema()->getCreateModelCallback();
 
-        return $creator ? $creator($request) : $this->resource->getAdapter()->create();
+        return $createModel ? $createModel($request) : $this->resource->getAdapter()->create();
     }
 
     private function fillDefaultValues(array &$data, Request $request)
diff --git a/src/Handler/Delete.php b/src/Handler/Delete.php
index 8452f9a..16c89bf 100644
--- a/src/Handler/Delete.php
+++ b/src/Handler/Delete.php
@@ -1,15 +1,24 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Handler;
 
 use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
 use Psr\Http\Server\RequestHandlerInterface;
-use function Tobyz\JsonApiServer\evaluate;
 use Tobyz\JsonApiServer\Exception\ForbiddenException;
 use Tobyz\JsonApiServer\ResourceType;
-use function Tobyz\JsonApiServer\run_callbacks;
 use Zend\Diactoros\Response\EmptyResponse;
+use function Tobyz\JsonApiServer\evaluate;
+use function Tobyz\JsonApiServer\run_callbacks;
 
 class Delete implements RequestHandlerInterface
 {
@@ -22,18 +31,23 @@ class Delete implements RequestHandlerInterface
         $this->model = $model;
     }
 
+    /**
+     * Handle a request to delete a resource.
+     *
+     * @throws ForbiddenException if the resource is not deletable.
+     */
     public function handle(Request $request): Response
     {
         $schema = $this->resource->getSchema();
 
-        if (! evaluate($schema->getDeletable(), [$this->model, $request])) {
+        if (! evaluate($schema->isDeletable(), [$this->model, $request])) {
             throw new ForbiddenException;
         }
 
         run_callbacks($schema->getListeners('deleting'), [$this->model, $request]);
 
-        if ($deleter = $this->resource->getSchema()->getDelete()) {
-            $deleter($this->model, $request);
+        if ($deleteCallback = $schema->getDeleteCallback()) {
+            $deleteCallback($this->model, $request);
         } else {
             $this->resource->getAdapter()->delete($this->model);
         }
diff --git a/src/Handler/Index.php b/src/Handler/Index.php
index 1ad6d85..8959f93 100644
--- a/src/Handler/Index.php
+++ b/src/Handler/Index.php
@@ -1,24 +1,32 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Handler;
 
-use Closure;
 use JsonApiPhp\JsonApi as Structure;
 use JsonApiPhp\JsonApi\Link\LastLink;
 use JsonApiPhp\JsonApi\Link\NextLink;
 use JsonApiPhp\JsonApi\Link\PrevLink;
-use JsonApiPhp\JsonApi\Meta;
 use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
 use Psr\Http\Server\RequestHandlerInterface;
-use Tobyz\JsonApiServer\JsonApi;
 use Tobyz\JsonApiServer\Exception\BadRequestException;
+use Tobyz\JsonApiServer\JsonApi;
 use Tobyz\JsonApiServer\JsonApiResponse;
 use Tobyz\JsonApiServer\ResourceType;
 use Tobyz\JsonApiServer\Schema\Attribute;
 use Tobyz\JsonApiServer\Schema\HasMany;
 use Tobyz\JsonApiServer\Schema\HasOne;
 use Tobyz\JsonApiServer\Serializer;
+use function Tobyz\JsonApiServer\run_callbacks;
 
 class Index implements RequestHandlerInterface
 {
@@ -33,35 +41,81 @@ class Index implements RequestHandlerInterface
         $this->resource = $resource;
     }
 
+    /**
+     * Handle a request to show a resource listing.
+     */
     public function handle(Request $request): Response
     {
-        $include = $this->getInclude($request);
-
-        $request = $this->extractQueryParams($request);
-
         $adapter = $this->resource->getAdapter();
         $schema = $this->resource->getSchema();
 
+        run_callbacks($schema->getListeners('listing'), [&$request]);
+
         $query = $adapter->query();
 
-        foreach ($schema->getScopes() as $scope) {
-            $request = $scope($query, $request, null) ?: $request;
+        run_callbacks($schema->getListeners('scope'), [$query, $request, null]);
+
+        $include = $this->getInclude($request);
+
+        $this->filter($query, $request);
+        $this->sort($query, $request);
+
+        $total = $schema->isCountable() ? $adapter->count($query) : null;
+
+        [$offset, $limit] = $this->paginate($query, $request);
+
+        $models = $adapter->get($query);
+
+        $this->loadRelationships($models, $include, $request);
+
+        run_callbacks($schema->getListeners('listed'), [$models, $request]);
+
+        $serializer = new Serializer($this->api, $request);
+
+        foreach ($models as $model) {
+            $serializer->add($this->resource, $model, $include);
         }
 
-        if ($filter = $request->getAttribute('jsonApiFilter')) {
-            $this->filter($query, $filter, $request);
+        return new JsonApiResponse(
+            new Structure\CompoundDocument(
+                new Structure\PaginatedCollection(
+                    new Structure\Pagination(...$this->buildPaginationLinks($request, $offset, $limit, count($models), $total)),
+                    new Structure\ResourceCollection(...$serializer->primary())
+                ),
+                new Structure\Included(...$serializer->included()),
+                new Structure\Link\SelfLink($this->buildUrl($request)),
+                new Structure\Meta('offset', $offset),
+                new Structure\Meta('limit', $limit),
+                ...($total !== null ? [new Structure\Meta('total', $total)] : [])
+            )
+        );
+    }
+
+    private function buildUrl(Request $request, array $overrideParams = []): string
+    {
+        [$selfUrl] = explode('?', $request->getUri(), 2);
+
+        $queryParams = array_replace_recursive($request->getQueryParams(), $overrideParams);
+
+        if (isset($queryParams['page']['offset']) && $queryParams['page']['offset'] <= 0) {
+            unset($queryParams['page']['offset']);
         }
 
-        $offset = $request->getAttribute('jsonApiOffset');
-        $limit = $request->getAttribute('jsonApiLimit');
-        $total = null;
+        if (isset($queryParams['filter'])) {
+            foreach ($queryParams['filter'] as $k => &$v) {
+                $v = $v === null ? '' : $v;
+            }
+        }
 
+        $queryString = http_build_query($queryParams);
+
+        return $selfUrl.($queryString ? '?'.$queryString : '');
+    }
+
+    private function buildPaginationLinks(Request $request, int $offset, ?int $limit, int $count, ?int $total)
+    {
         $paginationLinks = [];
-        $members = [
-            new Structure\Link\SelfLink($this->buildUrl($request)),
-            new Structure\Meta('offset', $offset),
-            new Structure\Meta('limit', $limit),
-        ];
+        $schema = $this->resource->getSchema();
 
         if ($offset > 0) {
             $paginationLinks[] = new Structure\Link\FirstLink($this->buildUrl($request, ['page' => ['offset' => 0]]));
@@ -77,165 +131,53 @@ class Index implements RequestHandlerInterface
             $paginationLinks[] = new PrevLink($this->buildUrl($request, $params));
         }
 
-        if ($schema->isCountable() && $schema->getPaginate()) {
-            $total = $adapter->count($query);
-
-            $members[] = new Meta('total', $total);
-
-            if ($offset + $limit < $total) {
-                $paginationLinks[] = new LastLink($this->buildUrl($request, ['page' => ['offset' => floor(($total - 1) / $limit) * $limit]]));
-            }
+        if ($schema->isCountable() && $schema->getPerPage() && $offset + $limit < $total) {
+            $paginationLinks[] = new LastLink($this->buildUrl($request, ['page' => ['offset' => floor(($total - 1) / $limit) * $limit]]));
         }
 
-        if ($sort = $request->getAttribute('jsonApiSort')) {
-            $this->sort($query, $sort, $request);
-        }
-
-        $this->paginate($query, $request);
-
-        $models = $adapter->get($query);
-
-        if ((count($models) === $limit && $total === null) || $offset + $limit < $total) {
+        if (($total === null && $count === $limit) || $offset + $limit < $total) {
             $paginationLinks[] = new NextLink($this->buildUrl($request, ['page' => ['offset' => $offset + $limit]]));
         }
 
-        $this->loadRelationships($models, $include, $request);
-
-        $serializer = new Serializer($this->api, $request);
-
-        foreach ($models as $model) {
-            $serializer->add($this->resource, $model, $include);
-        }
-
-        return new JsonApiResponse(
-            new Structure\CompoundDocument(
-                new Structure\PaginatedCollection(
-                    new Structure\Pagination(...$paginationLinks),
-                    new Structure\ResourceCollection(...$serializer->primary())
-                ),
-                new Structure\Included(...$serializer->included()),
-                ...$members
-            )
-        );
+        return $paginationLinks;
     }
 
-    private function buildUrl(Request $request, array $overrideParams = []): string
-    {
-        [$selfUrl] = explode('?', $request->getUri(), 2);
-        $queryParams = $request->getQueryParams();
-
-        $queryParams = array_replace_recursive($queryParams, $overrideParams);
-
-        if (isset($queryParams['page']['offset']) && $queryParams['page']['offset'] <= 0) {
-            unset($queryParams['page']['offset']);
-        }
-
-        if (isset($queryParams['filter'])) {
-            foreach ($queryParams['filter'] as $k => &$v) {
-                if ($v === null) {
-                    $v = '';
-                }
-            }
-        }
-
-        $queryString = http_build_query($queryParams);
-
-        return $selfUrl.($queryString ? '?'.$queryString : '');
-    }
-
-    private function extractQueryParams(Request $request): Request
+    private function sort($query, Request $request)
     {
         $schema = $this->resource->getSchema();
 
-        $queryParams = $request->getQueryParams();
-
-        $limit = $this->resource->getSchema()->getPaginate();
-
-        if (isset($queryParams['page']['limit'])) {
-            $limit = $queryParams['page']['limit'];
-
-            if ((! is_int($limit) && ! ctype_digit($limit)) || $limit < 1) {
-                throw new BadRequestException('page[limit] must be a positive integer', 'page[limit]');
-            }
-
-            $limit = min($this->resource->getSchema()->getLimit(), $limit);
+        if (! $sort = $request->getQueryParams()['sort'] ?? $schema->getDefaultSort()) {
+            return;
         }
 
-        $offset = 0;
-
-        if (isset($queryParams['page']['offset'])) {
-            $offset = $queryParams['page']['offset'];
-
-            if ((! is_int($offset) && ! ctype_digit($offset)) || $offset < 0) {
-                throw new BadRequestException('page[offset] must be a non-negative integer', 'page[offset]');
-            }
-        }
-
-        $request = $request
-            ->withAttribute('jsonApiLimit', $limit)
-            ->withAttribute('jsonApiOffset', $offset);
-
-        $sort = $queryParams['sort'] ?? $this->resource->getSchema()->getDefaultSort();
-
-        if ($sort) {
-            $sort = $this->parseSort($sort);
-            $fields = $schema->getFields();
-
-            foreach ($sort as $name => $direction) {
-                if (! isset($fields[$name])
-                    || ! $fields[$name] instanceof Attribute
-                    || ! $fields[$name]->getSortable()
-                ) {
-                    throw new BadRequestException("Invalid sort field [$name]", 'sort');
-                }
-            }
-        }
-
-        $request = $request->withAttribute('jsonApiSort', $sort);
-
-        $filter = $queryParams['filter'] ?? null;
-
-        if ($filter) {
-            if (! is_array($filter)) {
-                throw new BadRequestException('filter must be an array', 'filter');
-            }
-
-            $fields = $schema->getFields();
-
-            foreach ($filter as $name => $value) {
-                if ($name !== 'id' && (! isset($fields[$name]) || ! $fields[$name]->getFilterable())) {
-                    throw new BadRequestException("Invalid filter [$name]", "filter[$name]");
-                }
-            }
-        }
-
-        $request = $request->withAttribute('jsonApiFilter', $filter);
-
-        return $request;
-    }
-
-    private function sort($query, array $sort, Request $request)
-    {
-        $schema = $this->resource->getSchema();
         $adapter = $this->resource->getAdapter();
+        $sortFields = $schema->getSortFields();
+        $fields = $schema->getFields();
 
-        foreach ($sort as $name => $direction) {
-            $attribute = $schema->getFields()[$name];
-
-            if (($sorter = $attribute->getSortable()) instanceof Closure) {
-                $sorter($query, $direction, $request);
-            } else {
-                $adapter->sortByAttribute($query, $attribute, $direction);
+        foreach ($this->parseSort($sort) as $name => $direction) {
+            if (isset($sortFields[$name])) {
+                $sortFields[$name]($query, $direction, $request);
+                continue;
             }
+
+            if (
+                isset($fields[$name])
+                && $fields[$name] instanceof Attribute
+                && $fields[$name]->isSortable()
+            ) {
+                $adapter->sortByAttribute($query, $fields[$name], $direction);
+                continue;
+            }
+
+            throw new BadRequestException("Invalid sort field [$name]", 'sort');
         }
     }
 
     private function parseSort(string $string): array
     {
         $sort = [];
-        $fields = explode(',', $string);
 
-        foreach ($fields as $field) {
+        foreach (explode(',', $string) as $field) {
             if ($field[0] === '-') {
                 $field = substr($field, 1);
                 $direction = 'desc';
@@ -251,18 +193,51 @@ class Index implements RequestHandlerInterface
 
     private function paginate($query, Request $request)
     {
-        $limit = $request->getAttribute('jsonApiLimit');
-        $offset = $request->getAttribute('jsonApiOffset');
+        $schema = $this->resource->getSchema();
+        $queryParams = $request->getQueryParams();
+        $limit = $schema->getPerPage();
+
+        if (isset($queryParams['page']['limit'])) {
+            $limit = $queryParams['page']['limit'];
+
+            if (! ctype_digit(strval($limit)) || $limit < 1) {
+                throw new BadRequestException('page[limit] must be a positive integer', 'page[limit]');
+            }
+
+            $limit = min($schema->getLimit(), $limit);
+        }
+
+        $offset = 0;
+
+        if (isset($queryParams['page']['offset'])) {
+            $offset = $queryParams['page']['offset'];
+
+            if (! ctype_digit(strval($offset)) || $offset < 0) {
+                throw new BadRequestException('page[offset] must be a non-negative integer', 'page[offset]');
+            }
+        }
 
         if ($limit || $offset) {
             $this->resource->getAdapter()->paginate($query, $limit, $offset);
         }
+
+        return [$offset, $limit];
     }
 
-    private function filter($query, $filter, Request $request)
+    private function filter($query, Request $request)
     {
+        if (! $filter = $request->getQueryParams()['filter'] ?? null) {
+            return;
+        }
+
+        if (! is_array($filter)) {
+            throw new BadRequestException('filter must be an array', 'filter');
+        }
+
         $schema = $this->resource->getSchema();
         $adapter = $this->resource->getAdapter();
+        $filters = $schema->getFilters();
+        $fields = $schema->getFields();
 
         foreach ($filter as $name => $value) {
             if ($name === 'id') {
@@ -270,19 +245,25 @@ class Index implements RequestHandlerInterface
                 continue;
             }
 
-            $field = $schema->getFields()[$name];
-
-            if (($filter = $field->getFilterable()) instanceof Closure) {
-                $filter($query, $value, $request);
-            } elseif ($field instanceof Attribute) {
-                $adapter->filterByAttribute($query, $field, $value);
-            } elseif ($field instanceof HasOne) {
-                $value = explode(',', $value);
-                $adapter->filterByHasOne($query, $field, $value);
-            } elseif ($field instanceof HasMany) {
-                $value = explode(',', $value);
-                $adapter->filterByHasMany($query, $field, $value);
+            if (isset($filters[$name])) {
+                $filters[$name]($query, $value, $request);
+                continue;
             }
+
+            if (isset($fields[$name]) && $fields[$name]->isFilterable()) {
+                if ($fields[$name] instanceof Attribute) {
+                    $adapter->filterByAttribute($query, $fields[$name], $value);
+                } elseif ($fields[$name] instanceof HasOne) {
+                    $value = explode(',', $value);
+                    $adapter->filterByHasOne($query, $fields[$name], $value);
+                } elseif ($fields[$name] instanceof HasMany) {
+                    $value = explode(',', $value);
+                    $adapter->filterByHasMany($query, $fields[$name], $value);
+                }
+                continue;
+            }
+
+            throw new BadRequestException("Invalid filter [$name]", "filter[$name]");
         }
     }
 }
diff --git a/src/Handler/Show.php b/src/Handler/Show.php
index 4c55962..0e29ed7 100644
--- a/src/Handler/Show.php
+++ b/src/Handler/Show.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Handler;
 
 use JsonApiPhp\JsonApi\CompoundDocument;
@@ -11,6 +20,7 @@ use Tobyz\JsonApiServer\JsonApi;
 use Tobyz\JsonApiServer\JsonApiResponse;
 use Tobyz\JsonApiServer\ResourceType;
 use Tobyz\JsonApiServer\Serializer;
+use function Tobyz\JsonApiServer\run_callbacks;
 
 class Show implements RequestHandlerInterface
 {
@@ -27,15 +37,19 @@ class Show implements RequestHandlerInterface
         $this->model = $model;
     }
 
+    /**
+     * Handle a request to show a resource.
+     */
     public function handle(Request $request): Response
     {
         $include = $this->getInclude($request);
 
         $this->loadRelationships([$this->model], $include, $request);
 
-        $serializer = new Serializer($this->api, $request);
+        run_callbacks($this->resource->getSchema()->getListeners('show'), [$this->model, $request]);
 
-        $serializer->add($this->resource, $this->model, $include, true);
+        $serializer = new Serializer($this->api, $request);
+        $serializer->addSingle($this->resource, $this->model, $include);
 
         return new JsonApiResponse(
             new CompoundDocument(
diff --git a/src/Handler/Update.php b/src/Handler/Update.php
index 818d628..7671359 100644
--- a/src/Handler/Update.php
+++ b/src/Handler/Update.php
@@ -1,14 +1,23 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Handler;
 
 use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
 use Psr\Http\Server\RequestHandlerInterface;
-use function Tobyz\JsonApiServer\evaluate;
-use Tobyz\JsonApiServer\JsonApi;
 use Tobyz\JsonApiServer\Exception\ForbiddenException;
+use Tobyz\JsonApiServer\JsonApi;
 use Tobyz\JsonApiServer\ResourceType;
+use function Tobyz\JsonApiServer\evaluate;
 use function Tobyz\JsonApiServer\run_callbacks;
 
 class Update implements RequestHandlerInterface
@@ -26,15 +35,20 @@ class Update implements RequestHandlerInterface
         $this->model = $model;
     }
 
+    /**
+     * Handle a request to update a resource.
+     *
+     * @throws ForbiddenException if the resource is not updatable.
+     */
     public function handle(Request $request): Response
     {
         $schema = $this->resource->getSchema();
 
-        if (! evaluate($schema->getUpdatable(), [$this->model, $request])) {
+        if (! evaluate($schema->isUpdatable(), [$this->model, $request])) {
             throw new ForbiddenException;
         }
 
-        $data = $this->parseData($request->getParsedBody());
+        $data = $this->parseData($request->getParsedBody(), $this->model);
 
         $this->validateFields($data, $this->model, $request);
         $this->loadRelatedResources($data, $request);
@@ -47,6 +61,7 @@ class Update implements RequestHandlerInterface
 
         run_callbacks($schema->getListeners('updated'), [$this->model, $request]);
 
-        return (new Show($this->api, $this->resource, $this->model))->handle($request);
+        return (new Show($this->api, $this->resource, $this->model))
+            ->handle($request);
     }
 }
diff --git a/src/Http/MediaTypes.php b/src/Http/MediaTypes.php
index a9cceef..fc67a8b 100644
--- a/src/Http/MediaTypes.php
+++ b/src/Http/MediaTypes.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Http;
 
 class MediaTypes
diff --git a/src/JsonApi.php b/src/JsonApi.php
index a065094..d5d5f36 100644
--- a/src/JsonApi.php
+++ b/src/JsonApi.php
@@ -1,12 +1,21 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer;
 
-use Closure;
 use JsonApiPhp\JsonApi\ErrorDocument;
 use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
 use Psr\Http\Server\RequestHandlerInterface;
+use Tobyz\JsonApiServer\Adapter\AdapterInterface;
 use Tobyz\JsonApiServer\Exception\BadRequestException;
 use Tobyz\JsonApiServer\Exception\InternalServerErrorException;
 use Tobyz\JsonApiServer\Exception\MethodNotAllowedException;
@@ -31,16 +40,27 @@ final class JsonApi implements RequestHandlerInterface
         $this->baseUrl = $baseUrl;
     }
 
-    public function resource(string $type, $adapter, Closure $buildSchema = null): void
+    /**
+     * Define a new resource type.
+     */
+    public function resource(string $type, AdapterInterface $adapter, callable $buildSchema = null): void
     {
         $this->resources[$type] = new ResourceType($type, $adapter, $buildSchema);
     }
 
+    /**
+     * Get defined resource types.
+     */
     public function getResources(): array
     {
         return $this->resources;
     }
 
+    /**
+     * Get a resource type.
+     *
+     * @throws ResourceNotFoundException if the resource type has not been defined.
+     */
     public function getResource(string $type): ResourceType
     {
         if (! isset($this->resources[$type])) {
@@ -50,6 +70,15 @@ final class JsonApi implements RequestHandlerInterface
         return $this->resources[$type];
     }
 
+    /**
+     * Handle a request.
+     *
+     * @throws UnsupportedMediaTypeException if the request Content-Type header is invalid
+     * @throws NotAcceptableException if the request Accept header is invalid
+     * @throws MethodNotAllowedException if the request method is invalid
+     * @throws BadRequestException if the request URI is invalid
+     * @throws NotImplementedException
+     */
     public function handle(Request $request): Response
     {
         $this->validateRequest($request);
@@ -112,11 +141,7 @@ final class JsonApi implements RequestHandlerInterface
 
         $mediaTypes = new MediaTypes($header);
 
-        if ($mediaTypes->containsExactly('*/*')) {
-            return;
-        }
-
-        if ($mediaTypes->containsExactly(self::CONTENT_TYPE)) {
+        if ($mediaTypes->containsExactly('*/*') || $mediaTypes->containsExactly(self::CONTENT_TYPE)) {
             return;
         }
 
@@ -172,6 +197,12 @@ final class JsonApi implements RequestHandlerInterface
         }
     }
 
+    /**
+     * Convert an exception into a JSON:API error document response.
+     *
+     * If the exception is not an instance of ErrorProviderInterface, an
+     * Internal Server Error response will be produced.
+     */
     public function error($e)
     {
         if (! $e instanceof ErrorProviderInterface) {
@@ -188,6 +219,9 @@ final class JsonApi implements RequestHandlerInterface
         return new JsonApiResponse($data, $status);
     }
 
+    /**
+     * Get the base URL for the API.
+     */
     public function getBaseUrl(): string
     {
         return $this->baseUrl;
diff --git a/src/JsonApiResponse.php b/src/JsonApiResponse.php
index 85a1a01..6ce441b 100644
--- a/src/JsonApiResponse.php
+++ b/src/JsonApiResponse.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer;
 
 use Zend\Diactoros\Response\JsonResponse;
diff --git a/src/ResourceType.php b/src/ResourceType.php
index 09f403d..c23d27c 100644
--- a/src/ResourceType.php
+++ b/src/ResourceType.php
@@ -1,8 +1,16 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer;
 
-use Closure;
 use Tobyz\JsonApiServer\Adapter\AdapterInterface;
 use Tobyz\JsonApiServer\Schema\Type;
 
@@ -13,7 +21,7 @@ final class ResourceType
     private $buildSchema;
     private $schema;
 
-    public function __construct(string $type, AdapterInterface $adapter, Closure $buildSchema = null)
+    public function __construct(string $type, AdapterInterface $adapter, callable $buildSchema = null)
     {
         $this->type = $type;
         $this->adapter = $adapter;
diff --git a/src/Schema/Attribute.php b/src/Schema/Attribute.php
index c1608d9..2d5c20b 100644
--- a/src/Schema/Attribute.php
+++ b/src/Schema/Attribute.php
@@ -1,20 +1,38 @@
 <?php
 
-namespace Tobyz\JsonApiServer\Schema;
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
 
-use Closure;
+namespace Tobyz\JsonApiServer\Schema;
 
 final class Attribute extends Field
 {
     private $sortable = false;
 
-    public function sortable(Closure $callback = null)
+    public function getLocation(): string
     {
-        $this->sortable = $callback ?: true;
+        return 'attributes';
+    }
+
+    /**
+     * Allow this attribute to be used for sorting the resource listing.
+     */
+    public function sortable()
+    {
+        $this->sortable = true;
 
         return $this;
     }
 
+    /**
+     * Disallow this attribute to be used for sorting the resource listing.
+     */
     public function notSortable()
     {
         $this->sortable = false;
@@ -22,13 +40,8 @@ final class Attribute extends Field
         return $this;
     }
 
-    public function getSortable()
+    public function isSortable(): bool
     {
         return $this->sortable;
     }
-
-    public function getLocation(): string
-    {
-        return 'attributes';
-    }
 }
diff --git a/src/Schema/Concerns/HasListeners.php b/src/Schema/Concerns/HasListeners.php
index 82e25b8..ea5184f 100644
--- a/src/Schema/Concerns/HasListeners.php
+++ b/src/Schema/Concerns/HasListeners.php
@@ -13,9 +13,9 @@ namespace Tobyz\JsonApiServer\Schema\Concerns;
 
 trait HasListeners
 {
-    private $listeners = [];
+    protected $listeners = [];
 
-    public function getListeners(string $event)
+    public function getListeners(string $event): array
     {
         return $this->listeners[$event] ?? [];
     }
diff --git a/src/Schema/Field.php b/src/Schema/Field.php
index d1e885f..0f44136 100644
--- a/src/Schema/Field.php
+++ b/src/Schema/Field.php
@@ -1,10 +1,18 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Schema;
 
-use Closure;
-use function Tobyz\JsonApiServer\negate;
 use Tobyz\JsonApiServer\Schema\Concerns\HasListeners;
+use function Tobyz\JsonApiServer\negate;
 use function Tobyz\JsonApiServer\wrap;
 
 abstract class Field
@@ -16,10 +24,10 @@ abstract class Field
     private $visible = true;
     private $single = false;
     private $writable = false;
-    private $getter;
-    private $setter;
-    private $saver;
-    private $default;
+    private $getCallback;
+    private $setCallback;
+    private $saveCallback;
+    private $defaultCallback;
     private $filterable = false;
 
     public function __construct(string $name)
@@ -27,8 +35,15 @@ abstract class Field
         $this->name = $name;
     }
 
+    /**
+     * Get the location of the field within a JSON:API resource object
+     * ('attributes' or 'relationships').
+     */
     abstract public function getLocation(): string;
 
+    /**
+     * Set the model property to which this field corresponds.
+     */
     public function property(string $property)
     {
         $this->property = $property;
@@ -36,14 +51,20 @@ abstract class Field
         return $this;
     }
 
-    public function visible(Closure $condition = null)
+    /**
+     * Allow this field to be seen.
+     */
+    public function visible(callable $condition = null)
     {
         $this->visible = $condition ?: true;
 
         return $this;
     }
 
-    public function hidden(Closure $condition = null)
+    /**
+     * Disallow this field to be seen.
+     */
+    public function hidden(callable $condition = null)
     {
         $this->visible = $condition ? negate($condition) : false;
 
@@ -51,9 +72,11 @@ abstract class Field
     }
 
     /**
-     * Indicates that the field should only be visible on single root resources
+     * Only show this field on single root resources.
      *
-     * @return $this
+     * This is useful if a field requires an expensive calculation for each
+     * individual resource (eg. n+1 query problem). In this case it may be
+     * desirable to only have the field show when viewing a single resource.
      */
     public function single()
     {
@@ -62,69 +85,109 @@ abstract class Field
         return $this;
     }
 
-    public function writable(Closure $condition = null)
+    /**
+     * Allow this field to be written.
+     */
+    public function writable(callable $condition = null)
     {
         $this->writable = $condition ?: true;
 
         return $this;
     }
 
-    public function readonly(Closure $condition = null)
+    /**
+     * Disallow this field to be written.
+     */
+    public function readonly(callable $condition = null)
     {
         $this->writable = $condition ? negate($condition) : false;
 
         return $this;
     }
 
+    /**
+     * Define the value of this field.
+     *
+     * If null, the adapter will be used to get the value of this field.
+     *
+     * @param null|string|callable $value
+     */
     public function get($value)
     {
-        $this->getter = wrap($value);
+        $this->getCallback = $value === null ? null : wrap($value);
 
         return $this;
     }
 
-    public function set(Closure $callback)
+    /**
+     * Set the callback to apply a new value for this field to the model.
+     *
+     * If null, the adapter will be used to set the field on the model.
+     */
+    public function set(?callable $callback)
     {
-        $this->setter = $callback;
+        $this->setCallback = $callback;
 
         return $this;
     }
 
-    public function save(Closure $callback)
+    /**
+     * Set the callback to save this field to the model.
+     *
+     * If specified, the adapter will NOT be used to set the field on the model.
+     */
+    public function save(?callable $callback)
     {
-        $this->saver = $callback;
+        $this->saveCallback = $callback;
 
         return $this;
     }
 
-    public function saved(Closure $callback)
+    /**
+     * Run a callback after this field has been saved.
+     */
+    public function onSaved(callable $callback)
     {
         $this->listeners['saved'][] = $callback;
 
         return $this;
     }
 
+    /**
+     * Set a default value for this field to be used when creating a resource.
+     *
+     * @param null|string|callable $value
+     */
     public function default($value)
     {
-        $this->default = wrap($value);
+        $this->defaultCallback = wrap($value);
 
         return $this;
     }
 
-    public function validate(Closure $callback)
+    /**
+     * Add a validation callback for this field.
+     */
+    public function validate(callable $callback)
     {
         $this->listeners['validate'][] = $callback;
 
         return $this;
     }
 
-    public function filterable(Closure $callback = null)
+    /**
+     * Allow this field to be used for filtering the resource listing.
+     */
+    public function filterable()
     {
-        $this->filterable = $callback ?: true;
+        $this->filterable = true;
 
         return $this;
     }
 
+    /**
+     * Disallow this field to be used for filtering the resource listing.
+     */
     public function notFilterable()
     {
         $this->filterable = false;
@@ -137,53 +200,47 @@ abstract class Field
         return $this->name;
     }
 
-    public function getProperty()
+    public function getProperty(): ?string
     {
         return $this->property;
     }
 
-    /**
-     * @return bool|Closure
-     */
-    public function getVisible()
+    public function isVisible()
     {
         return $this->visible;
     }
 
-    public function getSingle()
+    public function isSingle(): bool
     {
         return $this->single;
     }
 
-    public function getWritable()
+    public function isWritable()
     {
         return $this->writable;
     }
 
-    public function getGetter()
+    public function getGetCallback()
     {
-        return $this->getter;
+        return $this->getCallback;
     }
 
-    public function getSetter()
+    public function getSetCallback(): ?callable
     {
-        return $this->setter;
+        return $this->setCallback;
     }
 
-    public function getSaver()
+    public function getSaveCallback(): ?callable
     {
-        return $this->saver;
+        return $this->saveCallback;
     }
 
-    public function getDefault()
+    public function getDefaultCallback()
     {
-        return $this->default;
+        return $this->defaultCallback;
     }
 
-    /**
-     * @return bool|Closure
-     */
-    public function getFilterable()
+    public function isFilterable(): bool
     {
         return $this->filterable;
     }
diff --git a/src/Schema/HasMany.php b/src/Schema/HasMany.php
index 32fb2ff..9bfab44 100644
--- a/src/Schema/HasMany.php
+++ b/src/Schema/HasMany.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Schema;
 
 final class HasMany extends Relationship
diff --git a/src/Schema/HasOne.php b/src/Schema/HasOne.php
index b111c95..709f433 100644
--- a/src/Schema/HasOne.php
+++ b/src/Schema/HasOne.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Schema;
 
 use Doctrine\Common\Inflector\Inflector;
diff --git a/src/Schema/Meta.php b/src/Schema/Meta.php
index 153fa3f..b0691cb 100644
--- a/src/Schema/Meta.php
+++ b/src/Schema/Meta.php
@@ -1,19 +1,25 @@
 <?php
 
-namespace Tobyz\JsonApiServer\Schema;
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
 
-use Closure;
-use function Tobyz\JsonApiServer\wrap;
+namespace Tobyz\JsonApiServer\Schema;
 
 final class Meta
 {
     private $name;
     private $value;
 
-    public function __construct(string $name, $value)
+    public function __construct(string $name, callable $value)
     {
         $this->name = $name;
-        $this->value = wrap($value);
+        $this->value = $value;
     }
 
     public function getName(): string
@@ -21,7 +27,7 @@ final class Meta
         return $this->name;
     }
 
-    public function getValue(): Closure
+    public function getValue(): callable
     {
         return $this->value;
     }
diff --git a/src/Schema/Relationship.php b/src/Schema/Relationship.php
index 4a4f23f..f4bd54c 100644
--- a/src/Schema/Relationship.php
+++ b/src/Schema/Relationship.php
@@ -1,20 +1,32 @@
 <?php
 
-namespace Tobyz\JsonApiServer\Schema;
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
 
-use Closure;
-use function Tobyz\JsonApiServer\negate;
+namespace Tobyz\JsonApiServer\Schema;
 
 abstract class Relationship extends Field
 {
     private $type;
-    private $allowedTypes;
     private $linkage = false;
     private $links = true;
     private $loadable = true;
     private $includable = false;
-    private $scopes = [];
 
+    public function getLocation(): string
+    {
+        return 'relationships';
+    }
+
+    /**
+     * Set the resource type that this relationship is to.
+     */
     public function type($type)
     {
         $this->type = $type;
@@ -22,35 +34,52 @@ abstract class Relationship extends Field
         return $this;
     }
 
+    /**
+     * Define this as a polymorphic relationship.
+     */
     public function polymorphic(array $types = null)
     {
-        $this->type = null;
-        $this->allowedTypes = $types;
+        $this->type = $types;
 
         return $this;
     }
 
-    public function linkage(Closure $condition = null)
+    /**
+     * Show resource linkage for the relationship.
+     */
+    public function linkage()
     {
-        $this->linkage = $condition ?: true;
+        $this->linkage = true;
 
         return $this;
     }
 
-    public function noLinkage(Closure $condition = null)
+    /**
+     * Do not show resource linkage for the relationship.
+     */
+    public function noLinkage()
     {
-        $this->linkage = $condition ? negate($condition) : false;
+        $this->linkage = false;
 
         return $this;
     }
 
-    public function loadable(Closure $callback = null)
+    /**
+     * Allow the relationship data to be eager-loaded into the model collection.
+     *
+     * This is used to prevent the n+1 query problem. If null, the adapter will
+     * be used to eager-load relationship data into the model collection.
+     */
+    public function loadable(callable $callback = null)
     {
         $this->loadable = $callback ?: true;
 
         return $this;
     }
 
+    /**
+     * Do not eager-load relationship data into the model collection.
+     */
     public function notLoadable()
     {
         $this->loadable = false;
@@ -58,6 +87,9 @@ abstract class Relationship extends Field
         return $this;
     }
 
+    /**
+     * Allow the relationship data to be included in a compound document.
+     */
     public function includable()
     {
         $this->includable = true;
@@ -65,6 +97,9 @@ abstract class Relationship extends Field
         return $this;
     }
 
+    /**
+     * Do not allow the relationship data to be included in a compound document.
+     */
     public function notIncludable()
     {
         $this->includable = false;
@@ -72,16 +107,9 @@ abstract class Relationship extends Field
         return $this;
     }
 
-    public function getType(): ?string
-    {
-        return $this->type;
-    }
-
-    public function getAllowedTypes(): ?array
-    {
-        return $this->allowedTypes;
-    }
-
+    /**
+     * Show links for the relationship.
+     */
     public function links()
     {
         $this->links = true;
@@ -89,6 +117,9 @@ abstract class Relationship extends Field
         return $this;
     }
 
+    /**
+     * Do not show links for the relationship.
+     */
     public function noLinks()
     {
         $this->links = false;
@@ -96,20 +127,33 @@ abstract class Relationship extends Field
         return $this;
     }
 
-    public function getLinkage()
+    /**
+     * Apply a scope to the query to eager-load the relationship data.
+     */
+    public function scope(callable $callback)
+    {
+        $this->listeners['scope'][] = $callback;
+    }
+
+    public function getType()
+    {
+        return $this->type;
+    }
+
+    public function isLinkage(): bool
     {
         return $this->linkage;
     }
 
-    public function hasLinks(): bool
+    public function isLinks(): bool
     {
         return $this->links;
     }
 
     /**
-     * @return bool|Closure
+     * @return bool|callable
      */
-    public function getLoadable()
+    public function isLoadable()
     {
         return $this->loadable;
     }
@@ -118,19 +162,4 @@ abstract class Relationship extends Field
     {
         return $this->includable;
     }
-
-    public function getLocation(): string
-    {
-        return 'relationships';
-    }
-
-    public function scope(Closure $callback)
-    {
-        $this->scopes[] = $callback;
-    }
-
-    public function getScopes(): array
-    {
-        return $this->scopes;
-    }
 }
diff --git a/src/Schema/Type.php b/src/Schema/Type.php
index 43390b6..c31080b 100644
--- a/src/Schema/Type.php
+++ b/src/Schema/Type.php
@@ -1,10 +1,18 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer\Schema;
 
-use Closure;
-use function Tobyz\JsonApiServer\negate;
 use Tobyz\JsonApiServer\Schema\Concerns\HasListeners;
+use function Tobyz\JsonApiServer\negate;
 
 final class Type
 {
@@ -12,29 +20,49 @@ final class Type
 
     private $fields = [];
     private $meta = [];
-    private $paginate = 20;
+    private $filters = [];
+    private $sortFields = [];
+    private $perPage = 20;
     private $limit = 50;
     private $countable = true;
+    private $listable = true;
     private $defaultSort;
     private $defaultFilter;
-    private $scopes = [];
-    private $saver;
+    private $saveCallback;
+    private $createModelCallback;
     private $creatable = false;
-    private $create;
     private $updatable = false;
     private $deletable = false;
-    private $delete;
+    private $deleteCallback;
 
+    /**
+     * Add an attribute to the resource type.
+     *
+     * If an attribute has already been defined with this name, it will be
+     * returned. Otherwise, the field will be overwritten.
+     */
     public function attribute(string $name): Attribute
     {
         return $this->field(Attribute::class, $name);
     }
 
+    /**
+     * Add a has-one relationship to the resource type.
+     *
+     * If a has-one relationship has already been defined with this name, it
+     * will be returned. Otherwise, the field will be overwritten.
+     */
     public function hasOne(string $name): HasOne
     {
         return $this->field(HasOne::class, $name);
     }
 
+    /**
+     * Add a has-many relationship to the resource type.
+     *
+     * If a has-many relationship has already been defined with this name, it
+     * will be returned. Otherwise, the field will be overwritten.
+     */
     public function hasMany(string $name): HasMany
     {
         return $this->field(HasMany::class, $name);
@@ -49,14 +77,17 @@ final class Type
         return $this->fields[$name];
     }
 
-    public function removeField(string $name)
+    /**
+     * Remove a field from the resource type.
+     */
+    public function removeField(string $name): void
     {
         unset($this->fields[$name]);
-
-        return $this;
     }
 
     /**
+     * Get the resource type's fields.
+     *
      * @return Field[]
      */
     public function getFields(): array
@@ -64,240 +95,408 @@ final class Type
         return $this->fields;
     }
 
-    public function meta(string $name, $value)
+    /**
+     * Add a meta attribute to the resource type.
+     */
+    public function meta(string $name, callable $value): Meta
     {
         return $this->meta[$name] = new Meta($name, $value);
     }
 
-    public function removeMeta(string $name)
+    /**
+     * Remove a meta attribute from the resource type.
+     */
+    public function removeMeta(string $name): void
     {
         unset($this->meta[$name]);
-
-        return $this;
     }
 
+    /**
+     * Get the resource type's meta attributes.
+     *
+     * @return Meta[]
+     */
     public function getMeta(): array
     {
         return $this->meta;
     }
 
-    public function filter(string $name, Closure $callback)
+    /**
+     * Add a filter to the resource type.
+     */
+    public function filter(string $name, callable $callback): void
     {
-        $this->attribute($name)
-            ->hidden()
-            ->filterable($callback);
-
-        return $this;
+        $this->filters[$name] = $callback;
     }
 
-    public function paginate(int $perPage)
+    /**
+     * Get the resource type's filters.
+     */
+    public function getFilters(): array
     {
-        $this->paginate = $perPage;
+        return $this->filters;
     }
 
-    public function dontPaginate()
+    /**
+     * Add a sort field to the resource type.
+     */
+    public function sort(string $name, callable $callback): void
     {
-        $this->paginate = null;
+        $this->sortFields[$name] = $callback;
     }
 
-    public function getPaginate(): ?int
+    /**
+     * Get the resource type's sort fields.
+     */
+    public function getSortFields(): array
     {
-        return $this->paginate;
+        return $this->sortFields;
     }
 
-    public function limit(int $limit)
+    /**
+     * Paginate the listing of the resource type.
+     */
+    public function paginate(int $perPage): void
+    {
+        $this->perPage = $perPage;
+    }
+
+    /**
+     * Don't paginate the listing of the resource type.
+     */
+    public function dontPaginate(): void
+    {
+        $this->perPage = null;
+    }
+
+    /**
+     * Get the number of records to list per page, or null if the list should
+     * not be paginated.
+     */
+    public function getPerPage(): ?int
+    {
+        return $this->perPage;
+    }
+
+    /**
+     * Limit the maximum number of records that can be listed.
+     */
+    public function limit(int $limit): void
     {
         $this->limit = $limit;
     }
 
-    public function noLimit()
+    /**
+     * Allow unlimited records to be listed.
+     */
+    public function noLimit(): void
     {
         $this->limit = null;
     }
 
+    /**
+     * Get the maximum number of records that can be listed, or null if there
+     * is no limit.
+     */
     public function getLimit(): int
     {
         return $this->limit;
     }
 
-    public function countable()
+    /**
+     * Mark the resource type as countable.
+     */
+    public function countable(): void
     {
         $this->countable = true;
     }
 
-    public function uncountable()
+    /**
+     * Mark the resource type as uncountable.
+     */
+    public function uncountable(): void
     {
         $this->countable = false;
     }
 
+    /**
+     * Get whether or not the resource type is countable.
+     */
     public function isCountable(): bool
     {
         return $this->countable;
     }
 
-    public function scope(Closure $callback)
+    /**
+     * Apply a scope to the query to fetch record(s).
+     */
+    public function scope(callable $callback): void
     {
-        $this->scopes[] = $callback;
+        $this->listeners['scope'][] = $callback;
     }
 
-    public function getScopes(): array
+    /**
+     * Run a callback before a resource is shown.
+     */
+    public function onShowing(callable $callback): void
     {
-        return $this->scopes;
+        $this->listeners['showing'][] = $callback;
     }
 
-    public function create(?Closure $callback)
+    /**
+     * Run a callback when a resource is shown.
+     */
+    public function onShown(callable $callback): void
     {
-        $this->create = $callback;
+        $this->listeners['shown'][] = $callback;
     }
 
-    public function getCreator()
+    /**
+     * Allow the resource type to be listed.
+     */
+    public function listable(callable $condition = null): void
     {
-        return $this->create;
+        $this->listable = $condition ?: true;
     }
 
-    public function creatable(Closure $condition = null)
+    /**
+     * Disallow the resource type to be listed.
+     */
+    public function notListable(callable $condition = null): void
+    {
+        $this->listable = $condition ? negate($condition) : false;
+    }
+
+    /**
+     * Get whether or not the resource type is allowed to be listed.
+     */
+    public function isListable()
+    {
+        return $this->listable;
+    }
+
+    /**
+     * Run a callback before the resource type is listed.
+     */
+    public function onListing(callable $callback): void
+    {
+        $this->listeners['listing'][] = $callback;
+    }
+
+    /**
+     * Run a callback when the resource type is listed.
+     */
+    public function onListed(callable $callback): void
+    {
+        $this->listeners['listed'][] = $callback;
+    }
+
+    /**
+     * Set the callback to create a new model instance.
+     *
+     * If null, the adapter will be used to create new model instances.
+     */
+    public function createModel(?callable $callback): void
+    {
+        $this->createModelCallback = $callback;
+    }
+
+    /**
+     * Get the callback to create a new model instance.
+     */
+    public function getCreateModelCallback(): ?callable
+    {
+        return $this->createModelCallback;
+    }
+
+    /**
+     * Allow the resource type to be created.
+     */
+    public function creatable(callable $condition = null): void
     {
         $this->creatable = $condition ?: true;
-
-        return $this;
     }
 
-    public function notCreatable(Closure $condition = null)
+    /**
+     * Disallow the resource type to be created.
+     */
+    public function notCreatable(callable $condition = null): void
     {
         $this->creatable = $condition ? negate($condition) : false;
-
-        return $this;
     }
 
-    public function getCreatable()
+    /**
+     * Get whether or not the resource type is allowed to be created.
+     */
+    public function isCreatable()
     {
         return $this->creatable;
     }
 
-    public function creating(Closure $callback)
+    /**
+     * Run a callback before a resource is created.
+     */
+    public function onCreating(callable $callback): void
     {
         $this->listeners['creating'][] = $callback;
-
-        return $this;
     }
 
-    public function created(Closure $callback)
+    /**
+     * Run a callback after a resource has been created.
+     */
+    public function onCreated(callable $callback): void
     {
         $this->listeners['created'][] = $callback;
-
-        return $this;
     }
 
-    public function updatable(Closure $condition = null)
+    /**
+     * Allow the resource type to be updated.
+     */
+    public function updatable(callable $condition = null): void
     {
         $this->updatable = $condition ?: true;
-
-        return $this;
     }
 
-    public function notUpdatable(Closure $condition = null)
+    /**
+     * Disallow the resource type to be updated.
+     */
+    public function notUpdatable(callable $condition = null): void
     {
         $this->updatable = $condition ? negate($condition) : false;
-
-        return $this;
     }
 
-    public function getUpdatable()
+    /**
+     * Get whether or not the resource type is allowed to be updated.
+     */
+    public function isUpdatable()
     {
         return $this->updatable;
     }
 
-    public function updating(Closure $callback)
+    /**
+     * Run a callback before a resource has been updated.
+     */
+    public function onUpdating(callable $callback): void
     {
         $this->listeners['updating'][] = $callback;
-
-        return $this;
     }
 
-    public function updated(Closure $callback)
+    /**
+     * Run a callback after a resource has been updated.
+     */
+    public function onUpdated(callable $callback): void
     {
         $this->listeners['updated'][] = $callback;
-
-        return $this;
     }
 
-    public function save(?Closure $callback)
+    /**
+     * Set the callback to save a model instance.
+     *
+     * If null, the adapter will be used to save model instances.
+     */
+    public function save(?callable $callback): void
     {
-        $this->saver = $callback;
-
-        return $this;
+        $this->saveCallback = $callback;
     }
 
-    public function getSaver()
+    /**
+     * Get the callback to save a model instance.
+     */
+    public function getSaveCallback(): ?callable
     {
-        return $this->saver;
+        return $this->saveCallback;
     }
 
-    public function deletable(Closure $condition = null)
+    /**
+     * Allow the resource type to be deleted.
+     */
+    public function deletable(callable $condition = null): void
     {
         $this->deletable = $condition ?: true;
-
-        return $this;
     }
 
-    public function notDeletable(Closure $condition = null)
+    /**
+     * Disallow the resource type to be deleted.
+     */
+    public function notDeletable(callable $condition = null): void
     {
         $this->deletable = $condition ? negate($condition) : false;
-
-        return $this;
     }
 
-    public function getDeletable()
+    /**
+     * Get whether or not the resource type is allowed to be deleted.
+     */
+    public function isDeletable()
     {
         return $this->deletable;
     }
 
-    public function delete(?Closure $callback)
+    /**
+     * Set the callback to delete a model instance.
+     *
+     * If null, the adapter will be used to delete model instances.
+     */
+    public function delete(?callable $callback): void
     {
-        $this->delete = $callback;
-
-        return $this;
+        $this->deleteCallback = $callback;
     }
 
-    public function getDelete()
+    /**
+     * Get the callback to delete a model instance.
+     */
+    public function getDeleteCallback(): ?callable
     {
-        return $this->delete;
+        return $this->deleteCallback;
     }
 
-    public function deleting(Closure $callback)
+    /**
+     * Run a callback before a resource has been deleted.
+     */
+    public function onDeleting(callable $callback): void
     {
         $this->listeners['deleting'][] = $callback;
-
-        return $this;
     }
 
-    public function deleted(Closure $callback)
+    /**
+     * Run a callback after a resource has been deleted.
+     */
+    public function onDeleted(callable $callback): void
     {
         $this->listeners['deleted'][] = $callback;
-
-        return $this;
     }
 
-    public function defaultSort(?string $sort)
+    /**
+     * Set the default sort parameter value to be used if none is specified in
+     * the query string.
+     */
+    public function defaultSort(?string $sort): void
     {
         $this->defaultSort = $sort;
-
-        return $this;
     }
 
-    public function getDefaultSort()
+    /**
+     * Get the default sort parameter value to be used if none is specified in
+     * the query string.
+     */
+    public function getDefaultSort(): ?string
     {
         return $this->defaultSort;
     }
 
-    public function defaultFilter(?array $filter)
+    /**
+     * Set the default filter parameter value to be used if none is specified in
+     * the query string.
+     */
+    public function defaultFilter(?array $filter): void
     {
         $this->defaultFilter = $filter;
-
-        return $this;
     }
 
-    public function getDefaultFilter()
+    /**
+     * Get the default filter parameter value to be used if none is specified in
+     * the query string.
+     */
+    public function getDefaultFilter(): ?array
     {
         return $this->defaultFilter;
     }
diff --git a/src/Serializer.php b/src/Serializer.php
index 694f750..1800e9d 100644
--- a/src/Serializer.php
+++ b/src/Serializer.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer;
 
 use DateTime;
@@ -30,13 +39,18 @@ final class Serializer
         $this->request = $request;
     }
 
-    public function add(ResourceType $resource, $model, array $include, bool $single = false)
+    public function add(ResourceType $resource, $model, array $include, bool $single = false): void
     {
         $data = $this->addToMap($resource, $model, $include, $single);
 
         $this->primary[] = $data['type'].':'.$data['id'];
     }
 
+    public function addSingle(ResourceType $resource, $model, array $include): void
+    {
+        $this->add($resource, $model, $include, true);
+    }
+
     private function addToMap(ResourceType $resource, $model, array $include, bool $single = false)
     {
         $adapter = $resource->getAdapter();
@@ -69,11 +83,11 @@ final class Serializer
                 continue;
             }
 
-            if ($field->getSingle() && ! $single) {
+            if ($field->isSingle() && ! $single) {
                 continue;
             }
 
-            if (! evaluate($field->getVisible(), [$model, $this->request])) {
+            if (! evaluate($field->isVisible(), [$model, $this->request])) {
                 continue;
             }
 
@@ -81,12 +95,12 @@ final class Serializer
                 $value = $this->attribute($field, $model, $adapter);
             } elseif ($field instanceof Schema\Relationship) {
                 $isIncluded = isset($include[$name]);
-                $isLinkage = evaluate($field->getLinkage(), [$this->request]);
+                $isLinkage = evaluate($field->isLinkage(), [$this->request]);
 
                 if (! $isIncluded && ! $isLinkage) {
                     $value = $this->emptyRelationship($field, $resourceUrl);
                 } elseif ($field instanceof Schema\HasOne) {
-                    $value = $this->toOne($field, $model, $adapter, $isIncluded, $isLinkage, $include[$name] ?? [], $resourceUrl);
+                    $value = $this->toOne($field, $model, $adapter, $isIncluded, $isLinkage, $include[$name] ?? [], $resourceUrl, $single);
                 } elseif ($field instanceof Schema\HasMany) {
                     $value = $this->toMany($field, $model, $adapter, $isIncluded, $isLinkage, $include[$name] ?? [], $resourceUrl);
                 }
@@ -114,7 +128,7 @@ final class Serializer
 
     private function attribute(Attribute $field, $model, AdapterInterface $adapter): Structure\Attribute
     {
-        if ($getter = $field->getGetter()) {
+        if ($getter = $field->getGetCallback()) {
             $value = $getter($model, $this->request);
         } else {
             $value = $adapter->getAttribute($model, $field);
@@ -127,11 +141,11 @@ final class Serializer
         return new Structure\Attribute($field->getName(), $value);
     }
 
-    private function toOne(Schema\HasOne $field, $model, AdapterInterface $adapter, bool $isIncluded, bool $isLinkage, array $include, string $resourceUrl)
+    private function toOne(Schema\HasOne $field, $model, AdapterInterface $adapter, bool $isIncluded, bool $isLinkage, array $include, string $resourceUrl, bool $single = false)
     {
         $links = $this->getRelationshipLinks($field, $resourceUrl);
 
-        $value = $isIncluded ? (($getter = $field->getGetter()) ? $getter($model, $this->request) : $adapter->getHasOne($model, $field)) : ($isLinkage && $field->getLoadable() ? $adapter->getHasOne($model, $field, ['id']) : null);
+        $value = $isIncluded ? (($getter = $field->getGetCallback()) ? $getter($model, $this->request) : $adapter->getHasOne($model, $field, false)) : ($isLinkage ? $adapter->getHasOne($model, $field, true) : null);
 
         if (! $value) {
             return new Structure\ToNull(
@@ -141,7 +155,7 @@ final class Serializer
         }
 
         if ($isIncluded) {
-            $identifier = $this->addRelated($field, $value, $include);
+            $identifier = $this->addRelated($field, $value, $include, $single);
         } else {
             $identifier = $this->relatedResourceIdentifier($field, $value);
         }
@@ -156,10 +170,10 @@ final class Serializer
 
     private function toMany(Schema\HasMany $field, $model, AdapterInterface $adapter, bool $isIncluded, bool $isLinkage, array $include, string $resourceUrl)
     {
-        if ($getter = $field->getGetter()) {
+        if ($getter = $field->getGetCallback()) {
             $value = $getter($model, $this->request);
         } else {
-            $value = ($isLinkage || $isIncluded) ? $adapter->getHasMany($model, $field) : null;
+            $value = ($isLinkage || $isIncluded) ? $adapter->getHasMany($model, $field, false) : null;
         }
 
         $identifiers = [];
@@ -197,7 +211,7 @@ final class Serializer
 
     private function getRelationshipLinks(Relationship $field, string $resourceUrl): array
     {
-        if (! $field->hasLinks()) {
+        if (! $field->isLinks()) {
             return [];
         }
 
@@ -207,12 +221,12 @@ final class Serializer
         ];
     }
 
-    private function addRelated(Relationship $field, $model, array $include): ResourceIdentifier
+    private function addRelated(Relationship $field, $model, array $include, bool $single = false): ResourceIdentifier
     {
-        $relatedResource = $field->getType() ? $this->api->getResource($field->getType()) : $this->resourceForModel($model);
+        $relatedResource = is_string($field->getType()) ? $this->api->getResource($field->getType()) : $this->resourceForModel($model);
 
         return $this->resourceIdentifier(
-            $this->addToMap($relatedResource, $model, $include)
+            $this->addToMap($relatedResource, $model, $include, $single)
         );
     }
 
@@ -286,7 +300,7 @@ final class Serializer
     {
         $type = $field->getType();
 
-        $relatedResource = $type ? $this->api->getResource($type) : $this->resourceForModel($model);
+        $relatedResource = is_string($type) ? $this->api->getResource($type) : $this->resourceForModel($model);
 
         return $this->resourceIdentifier([
             'type' => $relatedResource->getType(),
diff --git a/src/functions.php b/src/functions.php
index 18d9fb8..65632a6 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -1,5 +1,14 @@
 <?php
 
+/*
+ * This file is part of tobyz/json-api-server.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
 namespace Tobyz\JsonApiServer;
 
 use Closure;
@@ -49,16 +58,3 @@ function set_value(array &$data, Field $field, $value)
 {
     $data[$field->getLocation()][$field->getName()] = $value;
 }
-
-function array_set(array $array, $key, $value)
-{
-    $keys = explode('.', $key);
-
-    while (count($keys) > 1) {
-        $array = &$array[array_shift($keys)];
-    }
-
-    $array[array_shift($keys)] = $value;
-
-    return $array;
-}