Revert "Revert "Merge pull request #8313 from alan/only_save_changed_has_one_objects""

This reverts commit e94e6c27af.

Conflicts:
	activerecord/CHANGELOG.md

The original commit was reverted only to be safe since #14407 were reported.
We don't have any proof we added a regression with the original commit
so reverting it now will give us more problem.

Closes #14407

activerecord/CHANGELOG.md

@@ -434,12 +434,6 @@
 
     *Cody Cutrer*, *Steve Rice*, *Rafael Mendonça Franca*
 
-*   Save `has_one` association even if the record doesn't changed.
-
-    Fixes #14407.
-
-    *Rafael Mendonça França*
-
 *   Use singular table name in generated migrations when
     `ActiveRecord::Base.pluralize_table_names` is `false`.
 
@@ -518,6 +512,12 @@
 
     *Troy Kruthoff*, *Lachlan Sylvester*
 
+*   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*
+
 *   Allow strings to specify the `#order` value.
 
     Example:

activerecord/lib/active_record/autosave_association.rb

@@ -381,15 +381,16 @@ module ActiveRecord
       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
               end

activerecord/test/cases/autosave_association_test.rb

@@ -683,10 +683,23 @@ class TestDestroyAsPartOfAutosaveAssociation < ActiveRecord::TestCase
       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"
+    assert @pirate.save
+    assert_equal "NewName", @pirate.ship.reload.name
+  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?