Assume we have two models: User and Image. User has one image and image belongs to user. The code below:
Now we want to add validation for image to check if user is there or not.
So by adding just one line whenever you are trying to save image object, it will fire a query with respect to the user_id to check weather that user exists or not. In case user doesn’t exits “image.save” with return an error.
Happy Hacking :)