ActiveModel-style (Rails 3) `errors.to_json` for your Rails 2 app

active_model_style_errors_to_json.rb

module ActiveRecord
  class Errors
    def to_json
      inject({}) { |hash, error|
        attribute, message = error

        hash[attribute] ||= []
        hash[attribute] << message
        hash
      }.to_json
    end
  end
end