class User(BaseModel):
     username = models.CharField(max_length=20)
     password = models.CharField(max_length=20)
     groups   = models.ManyToManyField('Group', related_name='groups', db_table='USERS_TO_GROUPS')

class Group(BaseModel):
     name  = models.CharField(max_length=20)
     users = ManyToManyField(User, related_name='users', db_table=u'USERS_TO_GROUPS')