atk4\data\Model::hasOne PHP Method

    public function hasOne($link, $defaults = [])
    {
        return $this->_hasReference($this->_default_class_hasOne, $link, $defaults);
    }

 public function testModelProperty()
 {
     $db = new Persistence();
     $user = new Model($db, ['table' => 'user']);
     $user->id = 1;
     $user->hasOne('order_id', ['model' => ['atk4/data/Model', 'table' => 'order']]);
     $o = $user->ref('order_id');
     $this->assertEquals('order', $o->table);
 }