How can I stipulate a document ref in a structure when the document isn't yet declared?

[Matt Bodman]

Hi, I'm sure this is a common problem.  It is dealt with in other MongoDB ODM's but I wish to do it in MongoKit.  Consider this example:

class User(Document):

    structure = dict(

        first_name=basestring,

        modified_by=User  # User is not yet defined

        )

Is it possible with MongoKit to to this kind of reference?

Cheers.