How to write custom validation in Ruby on Rails

Posted on 16 July 2006

Fortunately writing custom validation is really easy in Rails. In your model create a method called validate and in there you can place your custom validation checks. For example:

def validate
   errors.add_to_base "If you are attaching a file you must enter a label for it" if !attachment.blank? and attachment_label.blank?
end

This custom validation does a simple check to see if the user has attempted to upload an attachment and if they have, ensure that they have entered a label for it.

The errors.add_to_base part means the error message will be output along with any other error messages using

<%= error_messages_for :object %>

If you require your custom validation to be reused across your more than one ActiveRecord model you should take a look at Peter Marklund's article that looks at all the different ways this can be achieved.

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