Looking around for the way to make custom validation errors for ActiveRecord I’ve found a number of different solution, including some quite complex ones those rewrite the helper to get required behavior messages. But it seems that the simplest possible way to do it is to rewrite ActiveRecord::Errors#full_messages method.

The cons of this way is that you keep all standard functionality of ActiveRecord – that means that if you want to provide custom messages only for some validation and keep the old ones for all others, you can easily do it. Just prefix your own messages with ‘^’ character – and they’ll be printed without column name in front of them. Other messages will have that name just as usual.

It’s even implemented as a plugin (here is the clone at GitHub, and here is the svn original at RubyForge) but I’m not sure if two modified lines are really worth being separated into a plugin. Anyway, if you wish – just grab the plugin from GitHub/RubyForge, and if you don’t – you can put the following code into your initializers (something like RAILS_ROOT/config/initializers/active_record_errors.rb), it will rewrite ActiveRecord’s default method on Rails startup.

And if you want to change the prefix from ^ to anything else – just change it in CUSTOM_PREFIX constant there on line 2 (and it doesn’t have to be only one character – something like ‘!!’ or ‘::’ will do as well).

class ActiveRecord::Errors

  def full_messages(options = {})
    full_messages = []

    @errors.each_key do |attr|
      @errors[attr].each do |message|
        next unless message

        if attr == "base"
          full_messages << message
        elsif message.mb_chars[0..(CUSTOM_PREFIX.size-1)] == CUSTOM_PREFIX
          full_messages << message.mb_chars[(CUSTOM_PREFIX.size)..-1]
          #key = :"activerecord.att.#{@base.class.name.underscore.to_sym}.#{attr}"
          attr_name = @base.class.human_attribute_name(attr)
          full_messages << attr_name + ' ' + message