ActiveResource::Base#update only puts changed attributes

lib/active_resource/base.rb

@@ -1426,8 +1426,10 @@ def connection(refresh = false)
       # Update the resource on the remote service.
       def update
         run_callbacks :update do
-          connection.put(element_path(prefix_options), encode, self.class.headers).tap do |response|
-            load_attributes_from_response(response)
+          if changed_attributes.present?
+            connection.put(element_path(prefix_options), encode(only: changed_attributes.keys), self.class.headers).tap do |response|
+              load_attributes_from_response(response)
+            end
           end
         end
       end

test/cases/base_test.rb

@@ -907,9 +907,26 @@ def test_update_conflict
       mock.get "/people/2.json", {}, @david
       mock.put "/people/2.json", @default_request_headers, nil, 409
     end
-    assert_raise(ActiveResource::ResourceConflict) { Person.find(2).save }
+    assert_raise(ActiveResource::ResourceConflict) do
+      person = Person.find(2)
+      person.name = 'new name'
+      person.save
+    end
+  end
+
+  def test_update_with_no_changes_does_nothing
+    matz = Person.find(:first)
+    matz.class.connection.expects(:put).never
+    matz.save
   end
 
+  def test_update_with_changes_only_puts_changed_attributes
+    matz = Person.find(:first)
+    matz.name = 'Rick'
+    resp = ActiveResource::Response.new(@matz, 200)
+    matz.class.connection.expects(:put).with("/people/#{matz.id}.json", matz.encode(:only => :name), {}).returns(resp)
+    matz.save
+  end
 
   ######
   # update_attribute(s)(!)

test/cases/observing_test.rb

@@ -43,6 +43,7 @@ def test_create_fires_save_and_create_notifications
 
   def test_update_fires_save_and_update_notifications
     person = Person.find(1)
+    person.name = 'new name'
     person.save
     assert_equal [:before_save, :before_update, :after_update, :after_save], self.history
   end