Assume we have two models: User and Image. User has one image and image belongs to user. The code below:
class User < ActiveRecord::Base has_one :image end class Image < ActiveRecord::Base belongs_to :user end
Now we want to add validation for image to check if user is there or not.
class Image < ActiveRecord::Base belongs_to :user validates :user, :presence => true end
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 :)