# Extension of the ActiveRecord's Dirty module to enable
# tracking changes even after the record has been saved
# (as ActiveRecord's dirty changes are emptied on save).
#
#  User.send :include, DirtyMemento
#
#  user = User.find_by_name 'kares'
#
#  assert ! user.name_changed?
#  assert ! user.name_previously_changed?
#  user.name = 'kares7'
#  assert user.name_changed?
#  assert ! user.name_previously_changed?
#  user.save
#  assert ! user.name_changed?
#  assert user.name_previously_changed?
#
module DirtyMemento

    def self.included(base)
      base.attribute_method_suffix '_previously_changed?', '_was_previously'
      base.alias_method_chain :save,   :dirty_memento
      base.alias_method_chain :save!,  :dirty_memento
      base.alias_method_chain :update, :dirty_memento
      base.alias_method_chain :reload, :dirty_memento
    end

    # Do any attributes have unsaved changes?
    #   person.previously_changed? # => false
    def previously_changed?
      ! previously_changed_attributes.empty?
    end

    # List of attributes with unsaved changes.
    #   person.previously_changed # => ['name']
    def previously_changed
      previously_changed_attributes.keys
    end

    def save_with_dirty_memento(*args) #:nodoc:
      set_previously_changed_attributes
      save_without_dirty_memento(*args)
    end

    def save_with_dirty_memento!(*args) #:nodoc:
      set_previously_changed_attributes
      save_without_dirty_memento!(*args)
    end

    def update_with_dirty_memento(*args) #:nodoc:
      set_previously_changed_attributes
      update_without_dirty_memento(*args)
    end

    def reload_with_dirty_memento(*args) #:nodoc:
      set_previously_changed_attributes
      reload_without_dirty_memento(*args)
    end

    private

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

    def set_previously_changed_attributes
      @_previously_changed_attributes = changed_attributes.clone
    end

    # Handle *_previously_changed? for method_missing.
    def attribute_previously_changed?(attr)
      previously_changed_attributes.include?(attr)
    end

    # Handle *_was_previously for method_missing.
    def attribute_was_previously(attr)
      attribute_previously_changed?(attr) ? previously_changed_attributes[attr] : __send__(attr)
    end

end