module ActiveRecord
  # Track unsaved attribute changes.
  #
  # A newly instantiated object is unchanged:
  #   person = Person.find_by_name('uncle bob')
  #   person.changed?       # => false
  #
  # Change the name:
  #   person.name = 'Bob'
  #   person.changed?       # => true
  #   person.name_changed?  # => true
  #   person.name_was       # => 'uncle bob'
  #   person.name_change    # => ['uncle bob', 'Bob']
  #   person.name = 'Bill'
  #   person.name_change    # => ['uncle bob', 'Bill']
  #
  # Save the changes:
  #   person.save
  #   person.changed?       # => false
  #   person.name_changed?  # => false
  #
  # Assigning the same value leaves the attribute unchanged:
  #   person.name = 'Bill'
  #   person.name_changed?  # => false
  #   person.name_change    # => nil
  #
  # Which attributes have changed?
  #   person.name = 'bob'
  #   person.changed        # => ['name']
  #   person.changes        # => { 'name' => ['Bill', 'bob'] }
  module Dirty
    def self.included(base)
      base.attribute_method_suffix '_changed?', '_change', '_was'
      base.alias_method_chain :write_attribute, :dirty
      base.alias_method_chain :save,            :dirty
      base.alias_method_chain :save!,           :dirty
    end

    # Do any attributes have unsaved changes?
    #   person.changed? # => false
    #   person.name = 'bob'
    #   person.changed? # => true
    def changed?
      !changed_attributes.empty?
    end

    # List of attributes with unsaved changes.
    #   person.changed # => []
    #   person.name = 'bob'
    #   person.changed # => ['name']
    def changed
      changed_attributes.keys
    end

    # Map of changed attrs => [original value, new value]
    #   person.changes # => {}
    #   person.name = 'bob'
    #   person.changes # => { 'name' => ['bill', 'bob'] }
    def changes
      changed.inject({}) { |h, attr| h[attr] = attribute_change(attr); h }
    end


    # Clear changed attributes after they are saved.
    def save_with_dirty(*args) #:nodoc:
      save_without_dirty(*args)
    ensure
      changed_attributes.clear
    end

    # Clear changed attributes after they are saved.
    def save_with_dirty!(*args) #:nodoc:
      save_without_dirty!(*args)
    ensure
      changed_attributes.clear
    end

    private
      # Map of change attr => original value.
      def changed_attributes
        @changed_attributes ||= {}
      end


      # Wrap write_attribute to remember original attribute value.
      def write_attribute_with_dirty(attr, value)
        attr = attr.to_s

        # The attribute already has an unsaved change.
        unless changed_attributes.include?(attr)
          old = read_attribute(attr)

          # Remember the original value if it's different.
          changed_attributes[attr] = old unless old == value
        end

        # Carry on.
        write_attribute_without_dirty(attr, value)
      end


      # Handle *_changed? for method_missing.
      def attribute_changed?(attr)
        changed_attributes.include?(attr)
      end

      # Handle *_change for method_missing.
      def attribute_change(attr)
        [changed_attributes[attr], __send__(attr)] if attribute_changed?(attr)
      end

      # Handle *_was for method_missing.
      def attribute_was(attr)
        attribute_changed?(attr) ? changed_attributes[attr] : __send__(attr)
      end
  end
end