QSqlQuery::execute after an insert returns always failure due to UNIQUE constraint validation. Double insertion while using QSqlQuery

Ajesh Kumar T.T.

Issue
The following code block return failure due to UNIQUE constraint.

QString query =
QString("INSERT INTO UsedStID(StID,UsedID)VALUES(%1,%2)").arg(createdID).arg(stID);
QSqlQuery queryExecuter(query);
if(!queryExecuter.exec())
{
qDebug() << query <<"____" <<queryExecuter.lastError();
}

Solution
Instead of following line :

QSqlQuery queryExecuter(query);

Use like:

QSqlQuery queryExecuter;
queryExecuter.exec(query);

Reason
The reason for this issue is, when you are passing a query string to the constructor it will execute this query and return resulting query object that contains data fetched from the DB (inserting result/error string/real data ifselect is used). Please see an example here http://doc.qt.io/qt-5/qsqlquery.html#details
This code snippet:

QSqlQuery query("SELECT country FROM artist");
while (query.next()) {
QString country = query.value(0).toString();
doSomething(country);
}

will fetch the data from the artist table and creates a query object from which the one can retrieve data. It is small thing, but it is not mentioned in documentation.

Note: - This is created as per the communication Dmitry who is a Qt support engineer.

SGaist

Hi,

More precisely you are executing the query twice hence the error since you have a unique constraint in your table.

Ajesh Kumar T.T.

@SGaist
Yes. That is the conclusion.