Autosave existing records on HMT associations when the parent is new

To me it seems like this should only be the case if `autosave: true` is
set on the association. However, when implemented that way, it caused
issues with has many associations, where we have explicit tests stating
that child records are updated when the parent is new, even if autosave
is not set (presumably to update the parent id, but other changed
attributes would be persisted as well).

It's quirky, but at least we should be consistently quirky. This
constitutes a minor but subtle change in behavior, and therefore should
not be backported to 4.2 and earlier.

Fixes #19782

1 files changed, 10 insertions, 0 deletions

diff --git a/activerecord/test/cases/autosave_association_test.rb b/activerecord/test/cases/autosave_association_test.rb
index 8f0d7bd037..0e6f38cad6 100644
--- a/activerecord/test/cases/autosave_association_test.rb
+++ b/activerecord/test/cases/autosave_association_test.rb
@@ -1278,6 +1278,16 @@ module AutosaveAssociationOnACollectionAssociationTests
     assert_equal new_names, @pirate.reload.send(@association_name).map(&:name)
   end
 
+  def test_should_update_children_when_autosave_is_true_and_parent_is_new_but_child_is_not
+    parrot = Parrot.create!(name: "Polly")
+    parrot.name = "Squawky"
+    pirate = Pirate.new(parrots: [parrot], catchphrase: "Arrrr")
+
+    pirate.save!
+
+    assert_equal "Squawky", parrot.reload.name
+  end
+
   def test_should_automatically_validate_the_associated_models
     @pirate.send(@association_name).each { |child| child.name = '' }