Custom ActiveModel Validator

blacklist.rb

#coding: utf-8
module Blacklist
  class Blacklisting < ActiveModel::EachValidator
    def validate_each(record, attr, value)
      message = options[:message] || "Ops, esse nome já foi escolhido"
      against = options[:against] || []
      record.errors.add(attr, message) if against.include? value
    end
  end

  module Helpers
    def blacklist(*attr_names)
      validates_with Blacklisting, _merge_attributes(attr_names) 
    end
  end
end

module ActiveRecord
  class Base
    extend Blacklist::Helpers
  end
end