code: def charge(self, user, vault_id=None):
          
          assert self.is_in_vault(user)
          if vault_id:
              user_vault = self.get(user=user, vault_id=vault_id)
          else:
              user_vault = self.get(user=user)
docs: If vault_id is not passed this will assume that there is only one instane of user and vault_id in the db.