Adding ability to change hasMany relation to hasOne

Now I write

```php
public function getOrders()
{
     return $this->hasOne(Order::className(), ['id' => 'order_id']);
}

public function getLastOrder()
{
     $query = $this->getOrders()->orderBy('created_at DESC');
     $query->multiple = false;
     return $query;
}
```

but it would be better with chain

thanks.