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, 12 insertions, 5 deletions

diff --git a/activestorage/lib/active_storage/attached/model.rb b/activestorage/lib/active_storage/attached/model.rb
index ae7f0685f2..06864a846f 100644
--- a/activestorage/lib/active_storage/attached/model.rb
+++ b/activestorage/lib/active_storage/attached/model.rb
@@ -93,12 +93,19 @@ module ActiveStorage
           end
 
           def #{name}=(attachables)
-            attachment_changes["#{name}"] =
-              if attachables.nil? || Array(attachables).none?
-                ActiveStorage::Attached::Changes::DeleteMany.new("#{name}", self)
-              else
-                ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, attachables)
+            if ActiveStorage.replace_on_assign_to_many
+              attachment_changes["#{name}"] =
+                if attachables.nil? || Array(attachables).none?
+                  ActiveStorage::Attached::Changes::DeleteMany.new("#{name}", self)
+                else
+                  ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, attachables)
+                end
+            else
+              if !attachables.nil? || Array(attachables).any?
+                attachment_changes["#{name}"] =
+                  ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, #{name}.blobs + attachables)
               end
+            end
           end
         CODE