code: func (m *Manager) UpsertIdentifier(username, azp, iyoid string) error {
  	// Count the amount of iyoids we already have first
  	idObj, err := m.GetByUsernameAndAZP(username, azp)
  	if err != nil && !db.IsNotFound(err) {
  		return err
  	}
  	if db.IsNotFound(err) {
  		idObj = &Identifier{}
  	}
  	if len(idObj.IyoIDs) >= maxIdentifiers {
  		return ErrIDLimitReached
  	}
  
  	_, err = m.getIdentifierCollection().Upsert(bson.M{"username": username, "azp": azp}, bson.M{"$push": bson.M{"iyoids": iyoid}})
  	return err
  }
docs: // UpsertIdentifier adds a new iyoid to a mapping or creates a new mapping