When counter_cache on wrong side of association

Absentmindedly put a counter_cache declaration on the has_many instead of where it belongs (pun intended.)

Rails 5 will complain in the most cryptic way it possibly can, which is to raise the following exception

ActiveModel::MissingAttributeError: can't write unknown attribute `true`

If you get that error, now you know how to fix it. Good luck and godspeed.