Adds a validation method or block to the class. This is useful when overriding the #validate instance method becomes too unwieldly and you’re looking for more descriptive declaration of your validations.

This can be done with a symbol pointing to a method:

class Comment < ActiveRecord::Base
  validate :must_be_friends

  def must_be_friends
    errors.add_to_base("Must be friends to leave a comment") unless commenter.friend_of?(commentee)
  end
end

Or with a block which is passed the current record to be validated:

class Comment < ActiveRecord::Base
  validate do |comment|
    comment.must_be_friends
  end

  def must_be_friends
    errors.add_to_base("Must be friends to leave a comment") unless commenter.friend_of?(commentee)
  end
end

This usage applies to #validate_on_create and #validate_on_update as well.

Source Code

# File active_record/validations.rb, line 327
def validate(*methods, &block)
  methods << block if block_given?
  write_inheritable_set(:validate, methods)
end