Ensure has_one autosave association callbacks get called once

When saving a record, autosave adds callbacks to save its' associations.
Since the associations can have similar callbacks for the inverse,
endless loops could occur.

To prevent these endless loops, the callbacks for `has_many` and
`belongs_to` are defined as methods that only execute once.
This is implemented in the `define_non_cyclic_method` method.
However, this wasn't used for the `has_one` callbacks.

While `has_one` association callbacks didn't result in endless loops,
they could execute multiple times.
For example for a bidirectional `has_one` with autosave enabled,
the `save_has_one_association` gets called twice:

    class Pirate < ActiveRecord::Base
      has_one :ship, autosave: true

      def save_has_one_association(reflection)
        @count ||= 0
        @count += 1 if reflection.name == :ship
        super
      end
    end

    class Ship < ActiveRecord::Base
      belongs_to :pirate, autosave: true
    end

    pirate = Pirate.new(catchphrase: "Aye")
    pirate.build_ship(name: "Nights Dirty Lightning")
    pirate.save!
    # this returns 2 instead of 1.
    assert_equal 1, pirate.instance_variable_get(:@count)

This commit changes `has_one` autosave callbacks to be non-cyclic as
well. By doing this the autosave callback are made more consistent for
all 3 cases: `has_many`, `has_one` and `belongs_to`.

activerecord/CHANGELOG.md

@@ -1,3 +1,11 @@
+* Ensure has_one autosave association callbacks get called once.
+
+    Change the `has_one` autosave callback to be non cyclic as well.
+    By doing this the autosave callback are made more consistent for
+    all 3 cases: `has_many`, `has_one` and `belongs_to`.
+
+    *Petrik de Heus*
+
 *   Only update dirty attributes once for cyclic autosave callbacks.
 
     Instead of calling `changes_applied` everytime `save` is called,

activerecord/lib/active_record/autosave_association.rb

@@ -196,7 +196,7 @@ module ActiveRecord
             after_create save_method
             after_update save_method
           elsif reflection.has_one?
-            define_method(save_method) { save_has_one_association(reflection) } unless method_defined?(save_method)
+            define_non_cyclic_method(save_method) { save_has_one_association(reflection) }
             # Configures two callbacks instead of a single after_save so that
             # the model may rely on their execution order relative to its
             # own callbacks.

activerecord/test/cases/autosave_association_test.rb

@@ -87,6 +87,56 @@ class TestAutosaveAssociationsInGeneral < ActiveRecord::TestCase
     assert_predicate r, :valid?
   end
 
+  def test_autosave_collection_association_callbacks_get_called_once
+    ship_with_saving_stack = Class.new(Ship) do
+      def save_collection_association(reflection)
+        @count ||= 0
+        @count += 1 if reflection.name == :parts
+        super
+      end
+    end
+
+    ship = ship_with_saving_stack.new(name: "Nights Dirty Lightning")
+    ship.parts.build(name: "part")
+    ship.save!
+    assert_equal 1, ship.instance_variable_get(:@count)
+  end
+
+  def test_autosave_has_one_association_callbacks_get_called_once
+    # a bidirectional autosave is required to trigger multiple calls to
+    # save_has_one_association
+    assert Ship.reflect_on_association(:pirate).options[:autosave]
+    assert Pirate.reflect_on_association(:ship).options[:autosave]
+
+    pirate_with_saving_stack = Class.new(Pirate) do
+      def save_has_one_association(reflection)
+        @count ||= 0
+        @count += 1 if reflection.name == :ship
+        super
+      end
+    end
+
+    pirate = pirate_with_saving_stack.new(catchphrase: "Aye")
+    pirate.build_ship(name: "Nights Dirty Lightning")
+    pirate.save!
+    assert_equal 1, pirate.instance_variable_get(:@count)
+  end
+
+  def test_autosave_belongs_to_association_callbacks_get_called_once
+    ship_with_saving_stack = Class.new(Ship) do
+      def save_belongs_to_association(reflection)
+        @count ||= 0
+        @count += 1 if reflection.name == :pirate
+        super
+      end
+    end
+
+    ship = ship_with_saving_stack.new(name: "Nights Dirty Lightning")
+    ship.build_pirate(catchphrase: "Aye")
+    ship.save!
+    assert_equal 1, ship.instance_variable_get(:@count)
+  end
+
   def test_should_not_add_the_same_callbacks_multiple_times_for_has_one
     assert_no_difference_when_adding_callbacks_twice_for Pirate, :ship
   end