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...

• 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

• Thats how things should be ... simple

  alexey at 31 Aug 10 at 04:09

• Did not work for me. I created validator in a model, but go error on execution:

  class Mypage < ActiveRecord::Base
  def validate_test
  if @mypage.disp_text =~ /@/
  errors.add(:disp_test, "Yor text include @ sign -------")
  @mypage.disp_text = "----changed ------"
  
  end
  end
  
  validates_presence_of :disp_text, :background_picture, :played_sound
  validate_test
  
  end

  An this is the error I got:

  undefined local variable or method `validate_test' for #<class:0x3a30698>
  C:/Ruby187/lib/ruby/gems/1.8/gems/activerecord-2.3.4/lib/active_record/base.rb:1959:in `method_missing'

  Zalek Bloom at 24 Oct 10 at 09:56

• @Zalek

  The proper syntax for calling your custom validation, in this case 'validate_test', is to call the active record method (or if you are in rails 3, active model) 'validates' and pass in the name of your custom validation method as a symbol such as:

  class MyPage < ActiveRecord::Base
  validates :validate_test

  def validate_test
  unless # Your test logic here
  errors.add :your_attribute, "Error Message"
  end
  end

  end

  Mike at 02 Nov 10 at 19:01

• nice post

  batz at 02 Mar 11 at 07:43

• @Mike: The validates method is only available in Rails 3.

  Mark Wilden at 31 Mar 11 at 10:42

• nice explanation

  kousika at 20 Apr 11 at 00:24

• Rails 3 method uses locales -- making your work neater and modularized.

  Yasky at 26 Oct 11 at 10:38

• You can read more about the locales file for error styling here on the Rails Guides I18n API #5. Here is a post I wrote up on how to harness the locales file in error styling: http://igbanam.wordpress.com/2011/10/25/customizing-error-messages-in-rails/

  Yasky at 26 Oct 11 at 10:40