Merge data into existing belongsToMany associations.

When creating a new entity that is linked to existing belongsToMany
records, we should merge request data into the existing entities.
Failing to do so makes it hard to create a new record, link it to an
existing record and update the existing record and its associations at
the same time.

Refs #8082

src/ORM/Marshaller.php

@@ -318,9 +318,7 @@ protected function _belongsToMany(Association $assoc, array $data, $options = []
             $query->andWhere(function ($exp) use ($conditions) {
                 return $exp->or_($conditions);
             });
-        }
 
-        if (isset($query)) {
             $keyFields = array_keys($primaryKey);
 
             $existing = [];
@@ -337,8 +335,10 @@ protected function _belongsToMany(Association $assoc, array $data, $options = []
                     }
                 }
                 $key = implode(';', $key);
+
+                // Update existing record and child associations
                 if (isset($existing[$key])) {
-                    $records[$i] = $existing[$key];
+                    $records[$i] = $this->merge($existing[$key], $data[$i], $options);
                 }
             }
         }
@@ -351,6 +351,7 @@ protected function _belongsToMany(Association $assoc, array $data, $options = []
         }
 
         foreach ($records as $i => $record) {
+            // Update junction table data in _joinData.
             if (isset($data[$i]['_joinData'])) {
                 $joinData = $jointMarshaller->one($data[$i]['_joinData'], $nested);
                 $record->set('_joinData', $joinData);

tests/TestCase/ORM/MarshallerTest.php

@@ -115,7 +115,9 @@ public function setUp()
     {
         parent::setUp();
         $articles = TableRegistry::get('Articles');
-        $articles->belongsTo('Users');
+        $articles->belongsTo('Users', [
+            'foreignKey' => 'author_id'
+        ]);
         $articles->hasMany('Comments');
         $articles->belongsToMany('Tags');
 
@@ -652,7 +654,7 @@ public function testOneBelongsToManyJoinDataAssociatedWithIds()
      *
      * @return void
      */
-    public function testBelongsToManyWithMixedJoinData()
+    public function testOneBelongsToManyWithMixedJoinData()
     {
         $data = [
             'title' => 'My title',
@@ -683,6 +685,46 @@ public function testBelongsToManyWithMixedJoinData()
         $this->assertEquals(1, $result->tags[1]->_joinData->active);
     }
 
+    public function testOneBelongsToManyWithNestedAssociations()
+    {
+        $this->tags->belongsToMany('Articles');
+        $data = [
+            'name' => 'new tag',
+            'articles' => [
+                // This nested article exists, and we want to update it.
+                [
+                    'id' => 1,
+                    'title' => 'New tagged article',
+                    'body' => 'New tagged article',
+                    'user' => [
+                        'id' => 1,
+                        'username' => 'newuser'
+                    ],
+                    'comments' => [
+                        ['comment' => 'New comment', 'user_id' => 1],
+                        ['comment' => 'Second comment', 'user_id' => 1],
+                    ]
+                ]
+            ]
+        ];
+        $marshaller = new Marshaller($this->tags);
+        $tag = $marshaller->one($data, ['associated' => ['Articles.Users', 'Articles.Comments']]);
+
+        $this->assertNotEmpty($tag->articles);
+        $this->assertCount(1, $tag->articles);
+        $this->assertInstanceOf('Cake\ORM\Entity', $tag->articles[0]);
+        $this->assertSame('New tagged article', $tag->articles[0]->title);
+
+        $this->assertNotEmpty($tag->articles[0]->user);
+        $this->assertInstanceOf('Cake\ORM\Entity', $tag->articles[0]->user);
+        $this->assertSame('newuser', $tag->articles[0]->user->username);
+
+        $this->assertNotEmpty($tag->articles[0]->comments);
+        $this->assertCount(2, $tag->articles[0]->comments);
+        $this->assertInstanceOf('Cake\ORM\Entity', $tag->articles[0]->comments[0]);
+        $this->assertInstanceOf('Cake\ORM\Entity', $tag->articles[0]->comments[1]);
+    }
+
     /**
      * Test belongsToMany association with mixed data and _joinData
      *