I added this feature so that a Map of changed fields could be retrieved

after a model had been saved. This is useful in the after_save callback
when you need to know what fields changed. At present there is no way
to do this other than have code in the before_save callback that takes
a copy of the changes Map, which I thought was a bit messy.

Example.

  person = Person.find_by_name('bob')
  person.name = 'robert'
  person.changes # => {'name' => ['bob, 'robert']}
  person.save
  person.changes # => {}
  person.previous_changes # => {'name' => ['bob, 'robert']}
  person.reload
  person.previous_changes # => {}

Signed-off-by: Joshua Peek <josh@joshpeek.com>

3 files changed, 101 insertions, 2 deletions

diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index 624c3647ca..735c61df74 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -72,12 +72,26 @@ module ActiveModel
       changed.inject({}) { |h, attr| h[attr] = attribute_change(attr); h }
     end
 
+    # Map of attributes that were changed when the model was saved.
+    #   person.name # => 'bob'
+    #   person.name = 'robert'
+    #   person.save
+    #   person.previous_changes # => {'name' => ['bob, 'robert']}
+    def previous_changes
+      previously_changed_attributes
+    end
+
     private
       # Map of change <tt>attr => original value</tt>.
       def changed_attributes
         @changed_attributes ||= {}
       end
 
+      # Map of fields that were changed when the model was saved
+      def previously_changed_attributes
+        @previously_changed || {}
+      end
+
       # Handle <tt>*_changed?</tt> for +method_missing+.
       def attribute_changed?(attr)
         changed_attributes.include?(attr)
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index b6c4df2a49..4df0f1af69 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -19,6 +19,7 @@ module ActiveRecord
       # Attempts to +save+ the record and clears changed attributes if successful.
       def save_with_dirty(*args) #:nodoc:
         if status = save_without_dirty(*args)
+          @previously_changed = changes
           changed_attributes.clear
         end
         status
@@ -26,12 +27,18 @@ module ActiveRecord
 
       # Attempts to <tt>save!</tt> the record and clears changed attributes if successful.
       def save_with_dirty!(*args) #:nodoc:
-        save_without_dirty!(*args).tap { changed_attributes.clear }
+        save_without_dirty!(*args).tap do
+          @previously_changed = changes 
+          changed_attributes.clear 
+        end
       end
 
       # <tt>reload</tt> the record and clears changed attributes.
       def reload_with_dirty(*args) #:nodoc:
-        reload_without_dirty(*args).tap { changed_attributes.clear }
+        reload_without_dirty(*args).tap do
+          previously_changed_attributes.clear
+          changed_attributes.clear
+        end
       end
 
       private
diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index 74571d923a..f456d273fe 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -308,6 +308,84 @@ class DirtyTest < ActiveRecord::TestCase
     end
   end
 
+  def test_previous_changes
+    # original values should be in previous_changes
+    pirate = Pirate.new
+  
+    assert_equal Hash.new, pirate.previous_changes
+    pirate.catchphrase = "arrr"
+    pirate.save!
+  
+    assert_equal 4, pirate.previous_changes.size
+    assert_equal [nil, "arrr"], pirate.previous_changes['catchphrase']
+    assert_equal [nil, pirate.id], pirate.previous_changes['id']
+    assert_nil pirate.previous_changes['updated_on'][0]
+    assert_not_nil pirate.previous_changes['updated_on'][1]
+    assert_nil pirate.previous_changes['created_on'][0]
+    assert_not_nil pirate.previous_changes['created_on'][1]
+    assert !pirate.previous_changes.key?('parrot_id')
+  
+    # original values should be in previous_changes
+    pirate = Pirate.new
+  
+    assert_equal Hash.new, pirate.previous_changes
+    pirate.catchphrase = "arrr"
+    pirate.save
+  
+    assert_equal 4, pirate.previous_changes.size
+    assert_equal [nil, "arrr"], pirate.previous_changes['catchphrase']
+    assert_equal [nil, pirate.id], pirate.previous_changes['id']
+    assert pirate.previous_changes.include?('updated_on')
+    assert pirate.previous_changes.include?('created_on')
+    assert !pirate.previous_changes.key?('parrot_id')
+  
+    pirate.catchphrase = "Yar!!"
+    pirate.reload
+    assert_equal Hash.new, pirate.previous_changes
+
+    pirate = Pirate.find_by_catchphrase("arrr")
+    pirate.catchphrase = "Me Maties!"
+    pirate.save!
+
+    assert_equal 2, pirate.previous_changes.size
+    assert_equal ["arrr", "Me Maties!"], pirate.previous_changes['catchphrase']
+    assert_not_nil pirate.previous_changes['updated_on'][0]
+    assert_not_nil pirate.previous_changes['updated_on'][1]
+    assert !pirate.previous_changes.key?('parrot_id')
+    assert !pirate.previous_changes.key?('created_on')
+
+    pirate = Pirate.find_by_catchphrase("Me Maties!")
+    pirate.catchphrase = "Thar She Blows!"
+    pirate.save
+
+    assert_equal 2, pirate.previous_changes.size
+    assert_equal ["Me Maties!", "Thar She Blows!"], pirate.previous_changes['catchphrase']
+    assert_not_nil pirate.previous_changes['updated_on'][0]
+    assert_not_nil pirate.previous_changes['updated_on'][1]
+    assert !pirate.previous_changes.key?('parrot_id')
+    assert !pirate.previous_changes.key?('created_on')
+
+    pirate = Pirate.find_by_catchphrase("Thar She Blows!")
+    pirate.update_attributes(:catchphrase => "Ahoy!")
+
+    assert_equal 2, pirate.previous_changes.size
+    assert_equal ["Thar She Blows!", "Ahoy!"], pirate.previous_changes['catchphrase']
+    assert_not_nil pirate.previous_changes['updated_on'][0]
+    assert_not_nil pirate.previous_changes['updated_on'][1]
+    assert !pirate.previous_changes.key?('parrot_id')
+    assert !pirate.previous_changes.key?('created_on')
+
+    pirate = Pirate.find_by_catchphrase("Ahoy!")
+    pirate.update_attribute(:catchphrase, "Ninjas suck!")
+
+    assert_equal 2, pirate.previous_changes.size
+    assert_equal ["Ahoy!", "Ninjas suck!"], pirate.previous_changes['catchphrase']
+    assert_not_nil pirate.previous_changes['updated_on'][0]
+    assert_not_nil pirate.previous_changes['updated_on'][1]
+    assert !pirate.previous_changes.key?('parrot_id')
+    assert !pirate.previous_changes.key?('created_on')    
+  end
+
   private
     def with_partial_updates(klass, on = true)
       old = klass.partial_updates?