public function testRemoveRelation_HasOne()
{
// collections
$carsCollection = $this->database->getCollection('cars');
$enginesCollection = $this->database->getCollection('engines');
$carDocument = $carsCollection->createDocument(array('brand' => 'Nissan'))->save();
$engineDocument = $enginesCollection->createDocument(array('car_id' => $carDocument->getId()))->save();
$carDocument->removeRelation('engine');
$this->assertEmpty($enginesCollection->getDocumentDirectly($engineDocument->getId())->car_id);
}