Preserve existing attachment assignment behavior for upgraded apps

Assigning to a collection of attachments appends rather than replacing, as in 5.2. Existing 5.2 apps that rely on this behavior will no longer break when they're upgraded to 6.0.

For apps generated on 6.0 or newer, assigning replaces the existing attachments in the collection. #attach should be used to add new attachments to the collection without removing existing ones.

I expect that we'll deprecate the old behavior in 6.1.

Closes #36374.

1 files changed, 26 insertions, 0 deletions

diff --git a/activestorage/test/models/attached/many_test.rb b/activestorage/test/models/attached/many_test.rb
index 878e284049..39ddecb041 100644
--- a/activestorage/test/models/attached/many_test.rb
+++ b/activestorage/test/models/attached/many_test.rb
@@ -269,6 +269,24 @@ class ActiveStorage::ManyAttachedTest < ActiveSupport::TestCase
     end
   end
 
+  test "updating an existing record with attachments when appending on assign" do
+    append_on_assign do
+      @user.highlights.attach create_blob(filename: "funky.jpg"), create_blob(filename: "town.jpg")
+
+      assert_difference -> { @user.reload.highlights.count }, +2 do
+        @user.update! highlights: [ create_blob(filename: "whenever.jpg"), create_blob(filename: "wherever.jpg") ]
+      end
+
+      assert_no_difference -> { @user.reload.highlights.count } do
+        @user.update! highlights: [ ]
+      end
+
+      assert_no_difference -> { @user.reload.highlights.count } do
+        @user.update! highlights: nil
+      end
+    end
+  end
+
   test "attaching existing blobs to a new record" do
     User.new(name: "Jason").tap do |user|
       user.highlights.attach create_blob(filename: "funky.jpg"), create_blob(filename: "town.jpg")
@@ -538,4 +556,12 @@ class ActiveStorage::ManyAttachedTest < ActiveSupport::TestCase
       User.remove_method :highlights
     end
   end
+
+  private
+    def append_on_assign
+      ActiveStorage.replace_on_assign_to_many, previous = false, ActiveStorage.replace_on_assign_to_many
+      yield
+    ensure
+      ActiveStorage.replace_on_assign_to_many = previous
+    end
 end