I have a mongo collection that holds documents which may or may
not contain various fields. I'm trying to determine the correct
way to check for a field. Right now all I have been able to come
up with is trying to deserialize (Bson) or get (Json) the field
and catch the exception that occurs when it isn't valid.

I've tried comparing the type to Json.Type.Undefined but that's
not working for some reason.

What's the right way to check for the existence of a field in a
Mongo document?