diff --git a/src/Illuminate/Database/Eloquent/Relations/MorphTo.php b/src/Illuminate/Database/Eloquent/Relations/MorphTo.php
index 53d25cb4470a..262741f30cfb 100644
--- a/src/Illuminate/Database/Eloquent/Relations/MorphTo.php
+++ b/src/Illuminate/Database/Eloquent/Relations/MorphTo.php
@@ -231,8 +231,14 @@ protected function matchToMorphParents($type, Collection $results)
      */
     public function associate($model)
     {
+        if ($model instanceof Model) {
+            $foreignKey = $this->ownerKey && $model->{$this->ownerKey}
+                            ? $this->ownerKey
+                            : $model->getKeyName();
+        }
+
         $this->parent->setAttribute(
-            $this->foreignKey, $model instanceof Model ? $model->{$this->ownerKey ?: $model->getKeyName()} : null
+            $this->foreignKey, $model instanceof Model ? $model->{$foreignKey} : null
         );
 
         $this->parent->setAttribute(
diff --git a/tests/Database/DatabaseEloquentMorphToTest.php b/tests/Database/DatabaseEloquentMorphToTest.php
index 6895d3a656de..a2046c8e5f2f 100644
--- a/tests/Database/DatabaseEloquentMorphToTest.php
+++ b/tests/Database/DatabaseEloquentMorphToTest.php
@@ -134,13 +134,13 @@ public function testMorphToWithSpecifiedClassDefault()
     public function testAssociateMethodSetsForeignKeyAndTypeOnModel()
     {
         $parent = m::mock(Model::class);
-        $parent->shouldReceive('getAttribute')->once()->with('foreign_key')->andReturn('foreign.value');
+        $parent->shouldReceive('getAttribute')->with('foreign_key')->andReturn('foreign.value');
 
         $relation = $this->getRelationAssociate($parent);
 
         $associate = m::mock(Model::class);
-        $associate->shouldReceive('getAttribute')->once()->andReturn(1);
-        $associate->shouldReceive('getMorphClass')->once()->andReturn('Model');
+        $associate->shouldReceive('getAttribute')->andReturn(1);
+        $associate->shouldReceive('getMorphClass')->andReturn('Model');
 
         $parent->shouldReceive('setAttribute')->once()->with('foreign_key', 1);
         $parent->shouldReceive('setAttribute')->once()->with('morph_type', 'Model');