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

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Autosave existing records on a has many through association when the parent
+    is new.
+
+    Fixes #19782.
+
+    *Sean Griffin*
+
 *   Fixed a bug where uniqueness validations would error on out of range values,
     even if an validation should have prevented it from hitting the database.
 

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -38,12 +38,10 @@ module ActiveRecord
       def insert_record(record, validate = true, raise = false)
         ensure_not_nested
 
-        if record.new_record?
-          if raise
-            record.save!(:validate => validate)
-          else
-            return unless record.save(:validate => validate)
-          end
+        if raise
+          record.save!(:validate => validate)
+        else
+          return unless record.save(:validate => validate)
         end
 
         save_through_record(record)

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 = '' }