[storm] How do I determine what columns a Reference joins?

[Jason Baker]

How do I determine what columns are joined by a reference?  For example:

class A:
    ...
    a_id = Integer(primary=True)

class B:
    ...
    a_id = Integer()
    A = Reference('a_id', A.a_id)

Given these classes, how can I programmatically determine that B.A
links B.a_id and A.a_id?  It looks like most of the attributes I could
use for this are underscored (like Reference's _local_key and
_remote_key)