Relation tagged with through model doesn't work as expected

[frmdstryr]

With models like

class User(Model):
    groups = Relation(lambda: Group).tag(through=lambda: UserGroup)

class Group(Model):
   # ...

class UserGroup(Model):
    user = Instance(User)
    group = Instance(Group)

I should be able to just filter by User.objects.filter(group=group) which should figure out from the through model that it should filter by user_group.group == group