/cc @javan

What does your response from /rails/active_storage/direct_uploads look like in the Network inspector? For example, here's what I see:

HI! Here's what I see:

Do you have ActiveRecord::Base.include_root_in_json = true set in your application?

Do you have ActiveRecord::Base.include_root_in_json = true set in your application?

Yes

Thanks, that's the issue. It should work if you set it to false, which is the Rails default:

We'll need to update the controller to ensure the response isn't wrapped. Any objections to this change, @georgeclaghorn?

--- a/activestorage/app/controllers/active_storage/direct_uploads_controller.rb
+++ b/activestorage/app/controllers/active_storage/direct_uploads_controller.rb
@@ -15,7 +15,7 @@ def blob_args
     end
 
     def direct_upload_json(blob)
-      blob.as_json(methods: :signed_id).merge(direct_upload: {
+      blob.as_json(root: false, methods: :signed_id).merge(direct_upload: {
         url: blob.service_url_for_direct_upload,
         headers: blob.service_headers_for_direct_upload
       })

No objection.

Leaving a comment here just in case. I ran into this error

undefined method `signed_id' for #<ActiveStorage::Attached::One:0x0000000008e27ec8>

for a model with an attachment on the Show view. The User had edited the item in question to replace the attachment. In my app the above JSON setting is set to the default. Not sure what can lead to this scenario. The User didn't report any issues, so I only realized when another User tripped across the Show view.

Looking at the active_storage_blobs the blob_id for the original file is there, but in the active_storage_attachments that blob_id is no longer there.

Upon further investigation, I believe the error I encountered matches the scenario in issue #31985 . The error message is what originally led me to this issue. Hopefully this will help someone else who stumbles across this page.