Add `ActiveModel::Dirty#[attr_name]_previously_changed?` and

`ActiveModel::Dirty#[attr_name]_previous_change` to improve access
to recorded changes after the model has been saved.

It makes the dirty-attributes query methods consistent before and after
saving.

activemodel/CHANGELOG.md

+*   Add `ActiveModel::Dirty#[attr_name]_previously_changed?` and
+    `ActiveModel::Dirty#[attr_name]_previous_change` to improve access
+    to recorded changes after the model has been saved.
+
+    It makes the dirty-attributes query methods consistent before and after
+    saving.
+
+    *Fernando Tapia Rico*
+
 *   Deprecate the `:tokenizer` option for `validates_length_of`, in favor of
     plain Ruby.
 

activemodel/lib/active_model/dirty.rb

@@ -76,9 +76,11 @@ module ActiveModel
   #
   # Reset the changes:
   #
-  #   person.previous_changes # => {"name" => ["Uncle Bob", "Bill"]}
+  #   person.previous_changes         # => {"name" => ["Uncle Bob", "Bill"]}
+  #   person.name_previously_changed? # => true
+  #   person.name_previous_change     # => ["Uncle Bob", "Bill"]
   #   person.reload!
-  #   person.previous_changes # => {}
+  #   person.previous_changes         # => {}
   #
   # Rollback the changes:
   #
@@ -115,6 +117,7 @@ module Dirty
 
     included do
       attribute_method_suffix '_changed?', '_change', '_will_change!', '_was'
+      attribute_method_suffix '_previously_changed?', '_previous_change'
       attribute_method_affix prefix: 'restore_', suffix: '!'
     end
 
@@ -179,6 +182,11 @@ def attribute_was(attr) # :nodoc:
       attribute_changed?(attr) ? changed_attributes[attr] : __send__(attr)
     end
 
+    # Handles <tt>*_previously_changed?</tt> for +method_missing+.
+    def attribute_previously_changed?(attr, options = {}) #:nodoc:
+      previous_changes_include?(attr)
+    end
+
     # Restore all previous data of the provided attributes.
     def restore_attributes(attributes = changed)
       attributes.each { |attr| restore_attribute! attr }
@@ -192,6 +200,12 @@ def changes_include?(attr_name)
       end
       alias attribute_changed_by_setter? changes_include?
 
+      # Returns +true+ if attr_name were changed before the model was saved,
+      # +false+ otherwise.
+      def previous_changes_include?(attr_name)
+        @previously_changed.include?(attr_name)
+      end
+
       # Removes current changes and makes them accessible through +previous_changes+.
       def changes_applied # :doc:
         @previously_changed = changes
@@ -209,6 +223,11 @@ def attribute_change(attr)
         [changed_attributes[attr], __send__(attr)] if attribute_changed?(attr)
       end
 
+      # Handles <tt>*_previous_change</tt> for +method_missing+.
+      def attribute_previous_change(attr)
+        @previously_changed[attr] if attribute_previously_changed?(attr)
+      end
+
       # Handles <tt>*_will_change!</tt> for +method_missing+.
       def attribute_will_change!(attr)
         return if attribute_changed?(attr)

activemodel/test/cases/dirty_test.rb

@@ -137,6 +137,19 @@ def reload
     assert_equal [nil, "Jericho Cane"], @model.previous_changes['name']
   end
 
+  test "setting new attributes should not affect previous changes" do
+    @model.name = "Jericho Cane"
+    @model.save
+    @model.name = "DudeFella ManGuy"
+    assert_equal [nil, "Jericho Cane"], @model.name_previous_change
+  end
+
+  test "saving should preserve model's previous changed status" do
+    @model.name = "Jericho Cane"
+    @model.save
+    assert @model.name_previously_changed?
+  end
+
   test "previous value is preserved when changed after save" do
     assert_equal({}, @model.changed_attributes)
     @model.name = "Paul"