I don't understand how it used to work

It did use to work, right?

Anyway, neither a scope nor a plain array liked [:tag_id], and since tag_ids were what we wanted to collect anyway, that's what we're doing now.

http://code.google.com/p/otwarchive/issues/detail?id=3673
Merge pull request #1286 from CristinaRO/098-issue-3673-tag-sets-error-500

I don't understand how it used to work