Handle dynamic fields

[emintham]

Dynamic fields such as expand/include fields defined in __init__ and elsewhere should be supported.

[emintham]

List of things needed to be supported here: https://docs.google.com/document/d/1yd4zF-b17z25uuFN8gs9jDK9_kg53NQpPwTsScodvVI/edit#heading=h.amsql57yzrwk