diff --git a/src/ResourceTrait.php b/src/ResourceTrait.php
index 9b4685c..ae26649 100644
--- a/src/ResourceTrait.php
+++ b/src/ResourceTrait.php
@@ -64,11 +64,27 @@ public function getResourceRelationships(array $linked = [])
         $keys = array_keys($resolvedFields);
 
         $relationships = array_fill_keys($keys, null);
+
+        $closureFields = array_filter($resolvedFields, function($value, $key) {
+            return is_callable($value);
+        }, ARRAY_FILTER_USE_BOTH);
+
+        foreach ($closureFields as $name => $value) {
+            $result = call_user_func($value, $this, $name);
+            if (isset($result['data'])) {
+                $relationships[$name] = $result['data'];
+            }
+        }
+
         $linkedFields = array_intersect($keys, $linked);
 
         foreach ($linkedFields as $name) {
             $definition = $resolvedFields[$name];
-            $relationships[$name] = is_string($definition) ? $this->$definition : call_user_func($definition, $this, $name);
+            $result = is_string($definition) ? $this->$definition : call_user_func($definition, $this, $name);
+
+            $relationships[$name] = (is_array($result) && isset($result['resource']))
+                ? $result['resource']
+                : $result;
         }
 
         return $relationships;
diff --git a/tests/SerializerTest.php b/tests/SerializerTest.php
index eba491d..13f81f4 100644
--- a/tests/SerializerTest.php
+++ b/tests/SerializerTest.php
@@ -4,11 +4,29 @@
  */
 namespace tuyakhov\jsonapi\tests;
 
+use tuyakhov\jsonapi\ResourceIdentifierInterface;
 use tuyakhov\jsonapi\tests\data\ResourceModel;
 use tuyakhov\jsonapi\Serializer;
 use yii\base\InvalidValueException;
 use yii\data\ArrayDataProvider;
 
+class ResourceModelIdentifier implements ResourceIdentifierInterface {
+    private $_id;
+    private $_type;
+
+    public function __construct($id, $type) {
+        $this->_id = $id;
+        $this->_type = $type;
+    }
+    public function getId() {
+        return $this->_id;
+    }
+
+    public function getType() {
+        return $this->_type;
+    }
+}
+
 class SerializerTest extends TestCase
 {
     protected function setUp()
@@ -55,6 +73,145 @@ public function testSerializeModelErrors()
         ], $serializer->serialize($model));
     }
 
+    public function testSerializeModelDataAddingNoDataValuesIfNotSpecified()
+    {
+        $serializer = new Serializer();
+        ResourceModel::$id = 123;
+        $model = new ResourceModel();
+        $this->assertSame([
+            'data' => [
+                'id' => '123',
+                'type' => 'resource-models',
+                'attributes' => [
+                    'field1' => 'test',
+                    'field2' => 2,
+                ],
+                'links' => [
+                    'self' => ['href' => 'http://example.com/resource/123']
+                ]
+            ]
+        ], $serializer->serialize($model));
+
+        ResourceModel::$fields = ['first_name'];
+        ResourceModel::$extraFields = ['extraField1'];
+        $model->extraField1 = new ResourceModel();
+        $relationship = [
+            'links' => [
+                'self' => ['href' => 'http://example.com/resource/123/relationships/extra-field1'],
+                'related' => ['href' => 'http://example.com/resource/123/extra-field1'],
+            ]
+        ];
+        $resource = [
+            'id' => '123',
+            'type' => 'resource-models',
+            'attributes' => [
+                'first-name' => 'Bob',
+            ],
+            'relationships' => [
+                'extra-field1' => [
+                    'links' => $relationship['links']
+                ]
+            ],
+            'links' => [
+                'self' => ['href' => 'http://example.com/resource/123']
+            ]
+        ];
+        $expected = [
+            'data' => $resource
+        ];
+        $expected['data']['relationships']['extra-field1'] = $relationship;
+        $this->assertSame($expected, $serializer->serialize($model));
+    }
+
+    public function testSerializeModelDataAddDataValuesIfOptIn()
+    {
+        $serializer = new Serializer();
+        ResourceModel::$id = 123;
+        $model = new ResourceModel();
+        $this->assertSame([
+            'data' => [
+                'id' => '123',
+                'type' => 'resource-models',
+                'attributes' => [
+                    'field1' => 'test',
+                    'field2' => 2,
+                ],
+                'links' => [
+                    'self' => ['href' => 'http://example.com/resource/123']
+                ]
+            ]
+        ], $serializer->serialize($model));
+
+        ResourceModel::$fields = ['first_name'];
+        ResourceModel::$extraFields = [
+            'extraField1' => function() { return new ResourceModel(); },
+        ];
+
+        $model->extraField1 = function() { return new ResourceModel(); };
+        $relationship = [
+            'links' => [
+                'self' => ['href' => 'http://example.com/resource/123/relationships/extra-field1'],
+                'related' => ['href' => 'http://example.com/resource/123/extra-field1'],
+            ]
+        ];
+        $resource = [
+            'id' => '123',
+            'type' => 'resource-models',
+            'attributes' => [
+                'first-name' => 'Bob',
+            ],
+            'relationships' => [
+                'extra-field1' => [
+                    'links' => $relationship['links']
+                ]
+            ],
+            'links' => [
+                'self' => ['href' => 'http://example.com/resource/123']
+            ]
+        ];
+        $expected = [
+            'data' => $resource
+        ];
+        $expected['data']['relationships']['extra-field1'] = $relationship;
+        $this->assertSame($expected, $serializer->serialize($model));
+
+        ResourceModel::$extraFields = [
+            'extraField1' => function() {
+                return [
+                    'resource' => new ResourceModel(),
+                    'data' => new ResourceModelIdentifier( "123",'resource-models'),
+                ];
+            }
+        ];
+        $relationship = [
+            'data' => ['id' => '123', 'type' => 'resource-models'],
+            'links' => [
+                'self' => ['href' => 'http://example.com/resource/123/relationships/extra-field1'],
+                'related' => ['href' => 'http://example.com/resource/123/extra-field1'],
+            ]
+        ];
+        $resource = [
+            'id' => '123',
+            'type' => 'resource-models',
+            'attributes' => [
+                'first-name' => 'Bob',
+            ],
+            'relationships' => [
+                'extra-field1' => [
+                    'links' => $relationship['links']
+                ]
+            ],
+            'links' => [
+                'self' => ['href' => 'http://example.com/resource/123']
+            ]
+        ];
+        $expected = [
+            'data' => $resource
+        ];
+        $expected['data']['relationships']['extra-field1'] = $relationship;
+        $this->assertSame($expected, $serializer->serialize($model));
+    }
+
     public function testSerializeModelData()
     {
         $serializer = new Serializer();
@@ -108,6 +265,11 @@ public function testSerializeModelData()
         $expected['included'][] = $resource;
         $expected['data']['relationships']['extra-field1'] = $relationship;
         $this->assertSame($expected, $serializer->serialize($model));
+
+        $model->extraField1 = [
+            'resource' => new ResourceModel(),
+        ];
+        $this->assertSame($expected, $serializer->serialize($model));
     }
 
     public function testExpand()