Save has_one associations only if record has changes

Prevents save related callbacks such as `after_commit` being
triggered when `has_one` objects are already persisted and have no
changes.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Only save has_one associations if record has changes.
+    Previously after save related callbacks, such as `#after_commit`, were triggered when the has_one
+    object did not get saved to the db.
+
+    *Alan Kennedy*
+
 *   ActiveRecord::Base#attribute_for_inspect now truncates long arrays (more than 10 elements)
 
     *Jan Bernacki*

activerecord/lib/active_record/autosave_association.rb

@@ -377,14 +377,15 @@ def save_collection_association(reflection)
       def save_has_one_association(reflection)
         association = association_instance_get(reflection.name)
         record      = association && association.load_target
+
         if record && !record.destroyed?
           autosave = reflection.options[:autosave]
 
           if autosave && record.marked_for_destruction?
             record.destroy
-          else
+          elsif autosave != false
             key = reflection.options[:primary_key] ? send(reflection.options[:primary_key]) : id
-            if autosave != false && (autosave || new_record? || record_changed?(reflection, record, key))
+            if (autosave && record.changed_for_autosave?) || new_record? || record_changed?(reflection, record, key)
 
               unless reflection.through_reflection
                 record[reflection.foreign_key] = key

activerecord/test/cases/autosave_association_test.rb

@@ -626,10 +626,25 @@ def save(*args)
       end
     end
 
+    @ship.pirate.catchphrase = "Changed Catchphrase"
+
     assert_raise(RuntimeError) { assert !@pirate.save }
     assert_not_nil @pirate.reload.ship
   end
 
+  def test_should_save_changed_has_one_changed_object_if_child_is_saved
+    @pirate.ship.name = "NewName"
+    @pirate.ship.expects(:save).once.returns(true)
+
+    assert @pirate.save
+  end
+
+  def test_should_not_save_changed_has_one_unchanged_object_if_child_is_saved
+    @pirate.ship.expects(:save).never
+
+    assert @pirate.save
+  end
+
   # belongs_to
   def test_should_destroy_a_parent_association_as_part_of_the_save_transaction_if_it_was_marked_for_destroyal
     assert !@ship.pirate.marked_for_destruction?