code: func (m *Manager) SaveVirtualCurrency(username string, currency DigitalAssetAddress) error {
  	if err := m.RemoveVirtualCurrency(username, currency.Label); err != nil {
  		return err
  	}
  	return m.getUserCollection().Update(
  		bson.M{"username": username},
  		bson.M{"$push": bson.M{"digitalwallet": currency}})
  }
docs: // SaveVirtualCurrency save or update virtualcurrency along with its label