Hi phoneboy. I'm checking with engineering on this. I have a feeling that the product_categories.id constraint was added for a feature that never made it out of EA and wasn't removed from the documentation. I'll verify and see if there is a workaround -- maybe something like using parent.id instead. (That's just a guess -- I hven't had time to test yet.)
parent.id didn't work and, from the docs, refers to a specific message.
The product category wouldn't necessarily be used in a message.
And, it didn't work when I tried it in the query.
