public function testHasChildren()
 {
     $entity = new Entity();
     $childrenRelationIndex = $entity->getChildrenRelationIndex();
     $collection = new Collection([$entity, new Entity(), new Entity()]);
     $collection->get(0)->setRelation($childrenRelationIndex, new Collection([new Entity(), new Entity(), new Entity()]));
     $this->assertTrue($collection->hasChildren(0));
 }