I was trying to build a static site for a non-developer user that is essentially a catalog of products. Each "Product" has a list of variations, which has 2 fields: name and price. The relation widget in "Product" could only store name or price, not both, making it harder to build the select I wanted.

Also, I've run into trouble with simple things like setting an unique ID for each "Product" without typing it manually, like an incremental id. Datetime field with UNIX formater was the closest I found, but it was not foolproof enough.