Whenever I create join tables, I’m always tempted to add an ID column, in case I want to upgrade it to a :through association at some point. I’m also tempted to timestamp columns, so that I can know when the association was created.

Sadly, it just doesn’t work with has_and_belongs_to_many.

• If you add an ID column, Rails will choke as soon as you try to access it. IDs are not allowed!

• If you add timestamps, Rails will make all of your association records readonly.

If you want to have an ID and timestamps, you’ll just have to bite the bullet and create an association model and a has_many :through relationship.

Maybe now that I’ve written about it, I’ll remember it next time and save myself debugging mysterious "ActiveRecord::ReadOnlyRecord: ActiveRecord::ReadOnlyRecord" errors.