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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Comments left...

• Yuz, good stuff

  Pimp at 11 Mar 08 at 11:42

• thanx i was really getting anoyed by now. thanx again

  Usman at 27 Jul 08 at 23:46

• You also can add a message error by an especific attribute, like this:

  errors.add :name, ‘Taigo isnt a name’ if self.name == ‘Taigo’

  Tiago Albineli Motta at 31 Jul 08 at 23:05

• Thanks. Exactly what I needed. Found off Google. It was nice and concise.

  Thanks at 18 Sep 08 at 20:41

• <%= error_messages_for :object ><= error_messages_for :object ><= error_messages_for :object ><= error_messages_for :object ><= error_messages_for :object ><= error_messages_for :object %>

  <%= error_messages_for :object %> at 25 Feb 09 at 06:12

• Yuz, good stuff

  Pimp commented on 11 Mar 08 at 11:42 at 25 Feb 09 at 06:15

• Ha, nice… Simple, clean. I forgot about it for sec, you where the first Google-hit ;)

  Angelo Michel at 11 May 10 at 10:10

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