Don't include association conditions when marshalling.

When marshalling belongsToMany associations we should ignore association
conditions as they can easily contain conditions on the joint table, or
create impossible conditions. Instead we should honour the `prop._ids`
data and leave validating associated data up to the developer, as they
may want to display an error on invalid data.

Refs #5073

src/ORM/Marshaller.php

@@ -250,7 +250,7 @@ protected function _loadBelongsToMany($assoc, $ids) {
 			$filter = [$primaryKey[0] . ' IN' => $ids];
 		}
 
-		return $assoc->find()->where($filter)->toArray();
+		return $target->find()->where($filter)->toArray();
 	}
 
 /**

tests/TestCase/ORM/MarshallerTest.php

@@ -886,6 +886,37 @@ public function testMergeFromIdsWithAutoAssociation() {
 		$this->assertCount(3, $result->tags);
 	}
 
+/**
+ * Tests that merging data to an entity containing belongsToMany and _ids
+ * with additional association conditions works.
+ *
+ * @return void
+ */
+	public function testMergeBelongsToManyFromIdsWithConditions() {
+		$this->articles->belongsToMany('Tags', [
+			'conditions' => ['ArticleTags.article_id' => 1]
+		]);
+
+		$entity = new Entity([
+			'title' => 'No tags',
+			'body' => 'Some content here',
+			'tags' => []
+		]);
+
+		$data = [
+			'title' => 'Haz moar tags',
+			'tags' => ['_ids' => [1, 2, 3]]
+		];
+		$entity->accessible('*', true);
+		$marshall = new Marshaller($this->articles);
+		$result = $marshall->merge($entity, $data, ['associated' => ['Tags']]);
+
+		$this->assertCount(3, $result->tags);
+		$this->assertInstanceOf('Cake\ORM\Entity', $result->tags[0]);
+		$this->assertInstanceOf('Cake\ORM\Entity', $result->tags[1]);
+		$this->assertInstanceOf('Cake\ORM\Entity', $result->tags[2]);
+	}
+
 /**
  * Tests that merging data to an entity containing belongsToMany and _ids
  * will ignore empty values.