Sokil\Mongo\DocumentRelationTest::testAddRelation_Belongs PHP Method

    public function testAddRelation_Belongs()
    {
        // collections
        $carsCollection = $this->database->getCollection('cars');
        $wheelsCollection = $this->database->getCollection('wheels');
        // add documents
        $carDocument = $carsCollection->createDocument(array('brand' => 'Nissan'))->save();
        // add target documents
        $wheelDocument = $wheelsCollection->createDocument(array('diameter' => 30))->addRelation('car', $carDocument)->save();
        $this->assertEquals($carDocument->getId(), $wheelDocument->car_id);
    }