I pretty vastly [improved it](https://github.com/mozilla/zamboni/commit/0b8237b#L1R64) in 0b8237b, I think.

I originally copy/pasted the entire method from the superclass, but given the limited use case for what we're doing here, it seems safe to trim most of the fat. I added [a docstring](https://github.com/mozilla/zamboni/commit/0b8237b#L1R24) to the field class clearly indicating the limited use case.