Solved QT Deploy QPSQL Plugin

[thaidy]

Hello ,

I am using QT creator 7.0.0 and I am with a problem that I dont know what to do.
I have developed my application in the Debug mode and everything was normal when I run and compile.

Then, I tried do deploy, so I change to the release mode, compile and execute. The program execute normally. Then I go to the release folder created, and copy all dlls of the MingW64/bin and all plugins folders to the release. Then when I try to execute my .exe Its is okay, but the database do not open.
With the bin files and plugins, i tried to execute directly from Qt and suddently I get this error:

QSqlDatabase: QPSQL driver not loaded
QSqlDatabase: available drivers:
QSqlDatabase: an instance of QCoreApplication is required for loading driver plugins
Failed to connect.

I really dont know what to do.

Thanks in advance .

[hskoglund]

Hi, using static QSqlDatabase instances is a recipe for problems, what happens if you move the declaration into for example your MainWindow.h class:

...
QSqlDatabase DB;
...

and then do the addDatabase() call in MainWindow's constructor:

ui->setupUi(this);
DB = QSqlDatabase::addDatabase("QPSQL"):
...

[JonB]

@thaidy
Hello and welcome.

Start by setting environment variable QT_DEBUG_PLUGINS=1 in a Command Prompt and then running your program. It should produce diagnostic output, last few lines should indicate what cannot be found.

QSqlDatabase: an instance of QCoreApplication is required for loading driver plugins

That might imply your code is wrong, but may be a result of the previous problem. We will see.

[thaidy]

@JonB
Hello , thank you for the answer .

The QT_DEBUG_PLUGINS is already like that. But what I really dont understand is why works normally in the Debug mode, but in the release mode, after I include in the folder all the necessary dlls and plugins, the database connection plugin does not even show in the available drivers.

My compile output is :

15:59:15: Running steps for project untitled...
15:59:15: Configuration unchanged, skipping qmake step.
15:59:15: Starting: "C:\Qt\Tools\mingw1120_64\bin\mingw32-make.exe" -j8
C:\Qt\6.3.0\mingw_64\bin\qmake.exe -o Makefile ..\untitled\untitled.pro -spec win32-g++ "CONFIG+=qtquickcompiler"
C:/Qt/Tools/mingw1120_64/bin/mingw32-make -f Makefile.Release
mingw32-make[1]: Entering directory 'C:/Users/HP/Documents/build-untitled-Desktop_Qt_6_3_0_MinGW_64_bit-Release'
g++ -c -fno-keep-inline-dllexport -O2 -std=gnu++1z -Wall -Wextra -Wextra -fexceptions -mthreads -DUNICODE -D_UNICODE -DWIN32 -DMINGW_HAS_SECURE_API=1 -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_SQL_LIB -DQT_CORE_LIB -DQT_NEEDS_QMAIN -I../untitled -I. -I../../../../Qt/6.3.0/mingw_64/include -I../../../../Qt/6.3.0/mingw_64/include/QtWidgets -I../../../../Qt/6.3.0/mingw_64/include/QtGui -I../../../../Qt/6.3.0/mingw_64/include/QtSql -I../../../../Qt/6.3.0/mingw_64/include/QtCore -Irelease -I. -I/include -I../../../../Qt/6.3.0/mingw_64/mkspecs/win32-g++ -o release\mainwindow.o ..\untitled\mainwindow.cpp
g++ -Wl,-s -Wl,-subsystem,windows -mthreads -o release\untitled.exe release/main.o release/mainwindow.o release/orders.o release/qrc_recurso.o release/moc_mainwindow.o release/moc_orders.o C:\Qt\6.3.0\mingw_64\lib\libQt6Widgets.a C:\Qt\6.3.0\mingw_64\lib\libQt6Gui.a C:\Qt\6.3.0\mingw_64\lib\libQt6Sql.a C:\Qt\6.3.0\mingw_64\lib\libQt6Core.a -lmingw32 C:\Qt\6.3.0\mingw_64\lib\libQt6EntryPoint.a -lshell32
mingw32-make[1]: Leaving directory 'C:/Users/HP/Documents/build-untitled-Desktop_Qt_6_3_0_MinGW_64_bit-Release'
15:59:27: The process "C:\Qt\Tools\mingw1120_64\bin\mingw32-make.exe" exited normally.
15:59:27: Elapsed time: 00:12.

[thaidy]

@JonB

I used a qDebug() to print in my terminal the drivers in the QList

First, in the release mode, without the dlls, i have this 3 plugins:

After I copy to my release folder the bin dlls the plugins suddenly disappears

[hskoglund]

Hi, using static QSqlDatabase instances is a recipe for problems, what happens if you move the declaration into for example your MainWindow.h class:

...
QSqlDatabase DB;
...

and then do the addDatabase() call in MainWindow's constructor:

ui->setupUi(this);
DB = QSqlDatabase::addDatabase("QPSQL"):
...

[thaidy]

@hskoglund said in QT Deploy QPSQL Plugin:

DB = QSqlDatabase::addDatabase("QPSQL"):

IT WAS THAT, THANK YOU VERY MUCH!!

[JonB]

@thaidy said in QT Deploy QPSQL Plugin:

@hskoglund said in QT Deploy QPSQL Plugin:

what happens if you move the declaration into for example your MainWindow.h class:
QSqlDatabase DB;
DB = QSqlDatabase::addDatabase("QPSQL"):

IT WAS THAT, THANK YOU VERY MUCH!!

Glad this has solved problem, certainly better than some static variable.

However, this is still not right/advised. Per https://doc.qt.io/qt-5/qsqldatabase.html#details, red box warning:

Warning: It is highly recommended that you do not keep a copy of the QSqlDatabase around as a member of a class, as this will prevent the instance from being correctly cleaned up on shutdown. If you need to access an existing QSqlDatabase, it should be accessed with database(). If you chose to have a QSqlDatabase member variable, this needs to be deleted before the QCoreApplication instance is deleted, otherwise it may lead to undefined behavior.

Qt asks you to simply call
static QSqlDatabase QSqlDatabase::database(const QString &connectionName = QLatin1String(defaultConnection), bool open = true)
whenever you need to access a QSqlDatabase, and not to keep even a member variable QSqlDatabase around. You don't need it.