Draft implementation of ActiveModel::Transformations

address.rb

class Address

  include ActiveModel::Transformations

  attr_accessor :street, :phone

  transform :street do |name|
    name.split(' ').map(&:capitalize).join(' ')
  end

  transform :phone do |number|
    number.gsub('-','')
  end

  def initialize(attributes = {})
    attributes.each do |key, value|
      send "#{key}=", value
    end
  end

  def save
    run_callbacks :save
  end

end

transformations.rb

# See Mike Perham's proposal on ticket: https://rails.lighthouseapp.com/projects/8994/tickets/6477-activemodel-transformations

module ActiveModel

  # == Active Model Transformations
  #
  # Provides a set of callbacks to perform transformations on your model
  # attributes before being assigned.
  #
  # A minimal implementation could be:
  #
  #   class Person
  #     include ActiveModel::Transformations
  #
  #     attr_accessor :first_name, :phone_number
  # 
  #     transform :first_name do |name|
  #       name.capitalize
  #     end
  #
  #     transform :phone_number do |number|
  #       num.gsub(/\D/, '')
  #     end
  #
  #   end
  #
  # Which pre-processes attributes on assignment like this:
  #
  #   person = Person.new(:first_name => 'john', :phone_number => '93-788-43-42')
  #   person.save!
  #   person.first_name               # => 'John'
  #   person.phone_number             # => '937884342'
  #
  module Transformations
    extend ActiveSupport::Concern
    include ActiveSupport::Callbacks

    included do
      define_callbacks :save
      define_callbacks :transform
      set_callback(:save, :before, :run_transformations!)
    end

    module ClassMethods

      def transform(*args, &block)
        args.each do |arg|
          set_callback(:transform, arg) do |object|
            object.send(:"#{arg}=", block.call(object.send(arg)))
          end
        end
      end

      # TODO: Copy transformations on inheritance.
      def inherited(base)
        # . . .
        super
      end
    end

  protected

    def run_transformations!
      run_callbacks :transform
    end
  end
end

transformations_test.rb

# encoding: utf-8
require 'cases/helper'
require 'models/address'

class TransformationsTest < ActiveModel::TestCase

  def teardown
    Address.reset_callbacks(:transform)
    Address.reset_callbacks(:save)
  end

  def test_single_field_transformation
    a = Address.new(:street => "c sant joan", :phone => '93-788-42-42')
    a.save
    assert_equal 'C Sant Joan', a.street
    assert_equal '937884242', a.phone
  end

end