code: def new(cls, user, provider, federated_id):
          
          if cls.get_user(provider, federated_id):
              raise exceptions.AuthError("Federation already")
  
          return cls.create(user_id=user.id,
                            provider=provider,
                            federated_id=federated_id)
docs: Create a new login
          :param user: AuthUser
          :param provider: str - ie: facebook, twitter, ...
          :param federated_id: str - an id associated to provider
          :return: