Assume we have two models: User and Image. User has one image and image belongs to user. The code below:
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Now we want to add validation for image to check if user is there or not.
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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 :)