QT QPSQL table insert return column - postgresql

Question

Using Qt Creator 4.1.0 based on Qt 5.6.
I have a postgresql table that looks like this:
CREATE TABLE shelf (
bookid sequence,
title character varying(200),
author character varying(200),
publisher character varying(200),
isbn character varying(200),
genre character varying(200)
);
I have just done a successful insert
QSqlQuery que;
que.exec("insert into shelf(title) values('blah') returning bookid;");
The insert worked fine, how to I get returning bookid?

Answer 1

try:
while (que.next()) {
QString bookid = que.value(0).toString();
qDebug() << bookid;
}
?..

Answer 2

You have two ways to get the last insert id when using Qt and PostgreSQL:
Using lastInsertId(): This has the advantage of being portable (i.e. your code keeps working if you decide to change to another DBMS later on), but it might be unsafe if you had triggers in your database that fire upon insertion in your table (e.g. shelf) and insert other values in other tables (note that it is implemented by issuing the query SELECT lastval(); on PostgreSQL). When using this approach, your code should look something like this:
QSqlQuery query;
query.prepare("INSERT INTO some_table(col_name) VALUES(?);");
query.addBindValue("sth");
if(!query.exec()) //insert statement failed
qWarning() << "insert statement failed with error: "
<< query.lastError().databaseText();
int insertId = query.lastInsertId().toInt();
//do anything you want with the id
qDebug() << "id: " << insertId;
Using the RETURNING clause: This has the advantage of being safer in case you had the weird triggers mentioned above, but it is not portable (i.e. the insert statement won't execute if you ever decide to change to another DBMS because it is not standard SQL anymore). This way, your code should look something like this:
QSqlQuery query;
query.prepare("INSERT INTO some_table(col_name) VALUES(?) RETURNING id_col_name;");
query.addBindValue("sth");
if(!query.exec()) //insert statement failed
qWarning() << "insert statement failed with error: "
<< query.lastError().databaseText();
if(!query.next()) //returning clause failed
qWarning() << "returning clause did not return any data";
int insertId = query.value(0).toInt();
//do anything you want with the id
qDebug() << "id: " << insertId;