Hello,

As many of you are probably aware, Qt docs are quite clear about instances of QSqlDatabase and threads:

A connection can only be used from within the thread that created it. Moving connections between threads or creating queries from a different thread is not supported.
After migrating from raw MySQL libraries to a QSql-based approach, my application started behaving oddly. Evidently the raw MySQL does not have the cited constraint: my application indeed uses exactly two threads: the main thread, and a thread that receives sensor data. Under the current implementation, both threads do work with the database.

The following illustrates the current solution to the problem, which takes a lazy-loading approach. Everything appears to work, but I would welcome any and all constructive criticism.

Qt Code: 
  1. // Controller.h
  2.  
  3. private:
  4.     CustomDatabase * databaseConnectionForThread(const Qt::HANDLE threadID = QThread::currentThreadId());
  5.  
  6.     QHash<Qt::HANDLE, CustomDatabase *> dbConnectionsByThread;
  7.     QMutex threadMutex, sqlMutex;
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Qt Code: 
  1. // Controller.cpp
  2.  
  3. CustomDatabase * Controller::databaseConnectionForThread(const Qt::HANDLE threadID) {
  4.     QMutexLocker locker(&threadMutex);
  5.     static int numberForConnectionName = 0;
  6.     CustomDatabase *dbConnection;
  7.     if (dbConnectionsByThread.contains(threadID))
  8.         dbConnection = dbConnectionsByThread.value(threadID);
  9.     else {
  10.         dbConnection = CustomDatabase::create(QString::number(++numberForConnectionName), this); // create() initializes and opens the connection it returns
  11.         if (!dbConnection->isOpen())
  12.             exitWithError(QString("Controller::databaseConnectionForThread() - Failed: %1!").arg(dbConnection->lastError()));
  13.         dbConnectionsByThread.insert(threadID, dbConnection);
  14.     }
  15.     return dbConnection;
  16. }
  17.  
  18. void Controller::doStuffWithDatabase() {
  19.     QMutexLocker locker(&sqlMutex);
  20.     CustomDatabase *dbConnection = databaseConnectionForThread();
  21.     // do stuff
  22. }
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Thank you very much for your time.