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executing query after cloned connection is used in another thread

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  • SGaistS SGaist

    Using processEvents is a last resort mesure that should be avoided.

    U Offline
    U Offline
    user4592357
    wrote on last edited by user4592357
    #17

    @sgaist
    okay so it means i need to go back to my thread implementation
    here's what i have

    bool doWork(const QString &newVal)
    {
         for (each of selected table rows)
         {
              if (!table->updateRow(row, newVal))
                  return false;
              // QCoreApplication::processEvents();
         }
        return true;
    }
    bool Table::updateRow(int row, const QString &newVal)
    {
         m_thread.execute(updateQuery, row, newVal);
         //...
    }
    

    when the processEvents is commented, the gui freezes, since the call to updateRow is executed in gui thread.
    what should i do? put the for loop of doWork() in the thread? (the table is generic and in my case i need to update the database but another table might do something else where multithreaded update is not needed... although it won't hurt maybe)

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    • SGaistS Offline
      SGaistS Offline
      SGaist
      Lifetime Qt Champion
      wrote on last edited by
      #18

      How proficient are you in the threading field ?

      Because from the looks of it, you are directly calling methods on your thread class and expecting them to actually run in that other thread. It's not how it works.

      Interested in AI ? www.idiap.ch
      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

      U 1 Reply Last reply
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      • SGaistS SGaist

        How proficient are you in the threading field ?

        Because from the looks of it, you are directly calling methods on your thread class and expecting them to actually run in that other thread. It's not how it works.

        U Offline
        U Offline
        user4592357
        wrote on last edited by user4592357
        #19

        @sgaist
        sorry for no context. i create the database and then use slots to execute the queries (because i want to execute the queries on some event, which is when the user decided to modify the data in my table widget).
        roughly speaking, this is my implementation:

        #pragma once
        
        #include <QThread>
        #include <QDebug>
        #include <QtSql/QSqlError>
        #include <QtSql/QSqlDatabase>
        #include <QtSql/QSqlQuery>
        
        class queryExecutor : public QObject
        {
        	Q_OBJECT
        public:
        	explicit queryExecutor(QObject *parent = nullptr) noexcept;
        
        	void setDatabase(const QString &sDbPath);
        
        public slots:
        	void execute(const QString &query, const QHash<QString, QString> &mapBoundVals);
        
        signals:
        	void finished(bool success, const QList<QSqlRecord> &records);
        
        private:
        	QSqlDatabase m_db;
        };
        
        class QueryThread : public QThread
        {
        	Q_OBJECT
        public:
        	explicit QueryThread(const QString &sDbPath) noexcept;
        
        protected:
        	void run() override;
        
        signals:
        	void queryFinished(bool success, const QList<QSqlRecord> &records);
        
        public slots:
        	void execute(const QString &query, const QHash<QString, QString> &mapBoundVals);
        
        private:
        	queryExecutor m_queryExecutor;
        	QString m_sDbPath;
        };
        
        queryExecutor::queryExecutor(QObject *parent /* = nullptr */) noexcept
        {
        }
        
        void queryExecutor::setDatabase(const QString &sDbPath)
        {
        	// create a thread-specific connection
        	m_db = QSqlDatabase::addDatabase("QSQLITE", QString("%1_%2")
        		.arg(sDbPath)
        		.arg(QString::number((quint64)QThread::currentThreadId(), 16)));
        
        	qDebug() << "conn name" << QString("%1_%2")
        		.arg(sDbPath)
        		.arg(QString::number((quint64)QThread::currentThreadId(), 16));
        
        	m_db.setDatabaseName(sDbPath);
        
        	if (!m_db.open())
        	{
        		qDebug() << "db not open:" << m_db.lastError().text();
        	}
        }
        
        void queryExecutor::execute(const QString &query, const QHash<QString, QString> &mapBoundVals)
        {
        	QList<QSqlRecord> records;
        	QSqlQuery sqlQuery(m_db);
        	sqlQuery.prepare(query);
        
        	for (const auto &sPlaceholder : mapBoundVals.keys())
        	{
        		auto bound = mapBoundVals[sPlaceholder];
        		sqlQuery.bindValue(sPlaceholder, bound);
        	}
        
        	if (!sqlQuery.exec())
        	{
        		qDebug() << "query failed:" << sqlQuery.lastError().text();
        		emit finished(false, records);
        		return;
        	}
        
        	while (sqlQuery.next())
        		records.push_back(sqlQuery.record());
        
        	emit finished(true, records);
        }
        
        QueryThread::QueryThread(const QString &sDbPath) noexcept
        	: m_sDbPath(sDbPath)
        {
        }
        
        void QueryThread::execute(const QString &query, const QHash<QString, QString> &mapBoundVals)
        {
        	// forward the execution to the worker
        	m_queryExecutor.execute(query, mapBoundVals);
        }
        
        void QueryThread::run()
        {
        	qDebug() << "started thread" << QThread::currentThreadId();
        
        	m_queryExecutor.setDatabase(m_sDbPath);
        
        	// forward final signal
        	connect(&m_queryExecutor, &queryExecutor::finished, this, &QueryThread::queryFinished);
        
        	exec();
        }
        
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        • Christian EhrlicherC Offline
          Christian EhrlicherC Offline
          Christian Ehrlicher
          Lifetime Qt Champion
          wrote on last edited by
          #20

          Your function queryExecutor::execute() is not run in your thread since the slot from where it's called is a function from an object which lives in the main thread. I would suggest reading the QThread documentation on how to properly use Signals/Slots with QThread: https://doc.qt.io/qt-5/threads-qobject.html

          Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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          U 1 Reply Last reply
          1
          • Christian EhrlicherC Christian Ehrlicher

            Your function queryExecutor::execute() is not run in your thread since the slot from where it's called is a function from an object which lives in the main thread. I would suggest reading the QThread documentation on how to properly use Signals/Slots with QThread: https://doc.qt.io/qt-5/threads-qobject.html

            U Offline
            U Offline
            user4592357
            wrote on last edited by user4592357
            #21

            @christian-ehrlicher
            yeah that's right. query executor should execute the query since it is in the new thread, so the code should be like this:

            void QueryThread::execute(const QString &query, const QHash<QString, QString> &mapBoundVals)
            {
            	// forward the execution to the worker
            	emit executefwd(query, mapBoundVals);
            }
            
            void QueryThread::run()
            {
            	qDebug() << "started thread" << QThread::currentThreadId();
            
            	m_queryExecutor.setDatabase(m_sDbPath);
            
                    connect( this, SIGNAL( executefwd( const QString&, const QHash<QString, QString> & ) ),
                         m_queryExecutor, SLOT( execute( const QString&, const QHash<QString, QString> & ) ) );
            
            	// forward final signal
            	connect(&m_queryExecutor, &queryExecutor::finished, this, &QueryThread::queryFinished);
            
            	exec();
            }
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            • SGaistS Offline
              SGaistS Offline
              SGaist
              Lifetime Qt Champion
              wrote on last edited by
              #22

              Your executor still lives in the main thread as it is created at your custom thread subclass creation time.

              Interested in AI ? www.idiap.ch
              Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

              U 1 Reply Last reply
              1
              • SGaistS SGaist

                Your executor still lives in the main thread as it is created at your custom thread subclass creation time.

                U Offline
                U Offline
                user4592357
                wrote on last edited by
                #23

                @sgaist
                oh okay, so it should be newed in QueryThread::run()

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                • SGaistS Offline
                  SGaistS Offline
                  SGaist
                  Lifetime Qt Champion
                  wrote on last edited by
                  #24

                  It can be on the stack. It really depends on how you want to implement that stuff. You seem to be mixing the worker object approach with the subclass QThread approach.

                  Interested in AI ? www.idiap.ch
                  Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                  U 1 Reply Last reply
                  0
                  • SGaistS SGaist

                    It can be on the stack. It really depends on how you want to implement that stuff. You seem to be mixing the worker object approach with the subclass QThread approach.

                    U Offline
                    U Offline
                    user4592357
                    wrote on last edited by
                    #25

                    @sgaist
                    i followed this example for my implementation: https://www.linuxjournal.com/article/9602
                    but anyways i think thread is still not much helpful because my for loop still executes in main thread. am i right?

                    bool doWork(const QString &newVal)
                    {
                         for (each of selected table rows)
                         {
                              if (!table->updateRow(row, newVal))
                                  return false;
                         }
                        return true;
                    }
                    
                    bool Table::updateRow(int row, const QString &newVal)
                    {
                         //update in another thread
                    }
                    
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                    • SGaistS Offline
                      SGaistS Offline
                      SGaist
                      Lifetime Qt Champion
                      wrote on last edited by
                      #26

                      As already said, you are mixing two different technique the wrong way.

                      If you subclass QThread and re-implement run, then what is executed in run happens in the thread managed by QThread.

                      However, you created an instance of your worker object in the original thread since it's part of your QThread subclass and thus the slot will be executed in the original thread.

                      Interested in AI ? www.idiap.ch
                      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                      1 Reply Last reply
                      1

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