Threads and database connection

[camel]

The problem as I see it is in this line

db = QSqlDatabase::addDatabase("QMYSQL3");

db = QSqlDatabase::addDatabase("QMYSQL3");

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addDatabase is a static function, which implies that is does not care in which Database object it was called; and you do not specify a specific second argument, the connectionName. The defaultvalue for this argument is "QLatin1String( defaultConnection )".

This means all your database connections will have the same connectionName. Keep this in mind when you read the following warning, copied straigt from the docs:

Warning: If you add a database with the same name as an existing database, the new database will replace the old one. This will happen automatically if you call this function more than once without specifying connectionName.

The way I read that is: as soon as your second thread initializes its database object, it will create a new default connection and delete the old one. The SqlDatabase object in the first thread will point to the old now stale connection and should thus cease to work. Which is rather consistent with your symptoms.

Changing the creation of database objects to the above mentioned method (meaning assuring that each connection/thread gets their own connectionName) should work. (Hopefully, not fully tested as I said ;-)