// AssignTariff assigns selected tariff to user
func (m *Message) AssignTariff() Message {
    db := ConnectDB(m.DbConnString)

    // check if user already has the same tariff as required
    if userHasSameTariff(db, m) {
        return Message{Passed: false, Errors: "You already have tariff \"" + m.Tariff + "\" assigned to username \"" + m.Username + "\"."}
    }

    // check if requiered tariff exists
    if tariffNotExists(db, m) {
        return Message{Passed: false, Errors: "Tariff \"" + m.Tariff + "\" doesn't exist in tightring_tariffs table."}
    }

    //check if cluster has enough resources for required tariff
    if clusterOutOfResources(db, m) {
        return Message{Passed: false, Errors: "Not enough free resources for assigning tariff \"" + m.Tariff + "\" to username \"" + m.Username + "\""}
    }

    // check if user has more already used resources then required tariff allows
    if userHasTooManyResources(db, m) {
        return Message{Passed: false, Errors: "Username \"" + m.Username + "\" has more used resources than tariff \"" + m.Tariff + "\" allows. Please free resources before changing tariff."}
    }

    // actualize user's free resources according to newly applied tariff
    //TODO

    // actually change tariff in the tightring_users table
    if !changeTariff(db, m) {
        return Message{Passed: false, Errors: "Error assigning tariff to user(something wrong while processing query)"}
    }

    return Message{Passed: true, Errors: "Tariff \"" + m.Tariff + "\" successfully assigned to username \"" + m.Username + "\""}
}