code: func (m *Manager) SaveBank(u *User, bank BankAccount) error {
  	if err := m.RemoveBank(u, bank.Label); err != nil {
  		return err
  	}
  	return m.getUserCollection().Update(
  		bson.M{"username": u.Username},
  		bson.M{"$push": bson.M{"bankaccounts": bank}})
  }
docs: // SaveBank save or update bank account