Sokil\Mongo\DocumentRelationTest::testRemoveRelation_HasOne PHP Method

    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);
    }