Sokil\Mongo\DocumentRelationTest::testAddRelation_HasOne PHP Method

    public function testAddRelation_HasOne()
    {
        // collections
        $carsCollection = $this->database->getCollection('cars');
        $enginesCollection = $this->database->getCollection('engines');
        $engineDocument = $enginesCollection->createDocument(array('power' => 300))->save();
        $carDocument = $carsCollection->createDocument(array('brand' => 'Nissan'))->save()->addRelation('engine', $engineDocument);
        $this->assertEquals($carDocument->getId(), $engineDocument->car_id);
    }