Building an API with better errors display with JSON. Must be places in config/initializers

active_model_errors.rb

# -*- encoding : utf-8 -*-
ActiveModel::Errors.module_eval do
  def as_json options = nil
    @api_errors
  end

  def initialize(base)
    @base     = base
    @messages = ActiveSupport::OrderedHash.new
    @api_errors = []
  end

  def add(attribute, message = nil, options = {})
    message ||= :invalid
    error_symbol = message if message.is_a?(Symbol)

    if message.is_a?(Symbol)
      message = generate_message(attribute, message, options.except(*ActiveModel::Errors::CALLBACKS_OPTIONS))
    elsif message.is_a?(Proc)
      message = message.call
    end

    @api_errors << {:attribute => attribute, :error => error_symbol, :message => full_message(attribute, message)}

    self[attribute] << message
  end

  def full_message attribute, message
    if attribute == :base
      message
    else
      attr_name = attribute.to_s.gsub('.', '_').humanize
      attr_name = @base.class.human_attribute_name(attribute, :default => attr_name)

      I18n.t(:"errors.format", {
        :default   => "%{attribute} %{message}",
        :attribute => attr_name,
        :message   => message
      })
    end
  end
end