4. How to connect to the database correctly in QtWebApp

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How to connect to the database correctly in QtWebApp


  • Good afternoon.
    I do it on the QtWebApp server. Made the DataBase class to work with the database.
    If I open a connection to the base in the RequestHandler class in the constructor, the treatment to the base in the function

    void RequestHandler::service(HttpRequest& request, HttpResponse& response)

    a connection error is returned, and if I open a connection in this function, it works normally for one request, if there is 2 requests, while the first request is wooling the database, then an error is issued and the server crashes.

    07.06.2020 15:36:44.134 1 WARNING  0x20d4 QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

    Please tell me how and where to create an object of the DataBase class and open it so that everything works?

    0
  • Lifetime Qt Champion

    Hi,

    If you open a new connection for each request then you have to give them unique names and not forget to delete said connection at the end of the request handling.

    1

  • I created objects of the DataBase class this way.

        DataBase db;
    db.openDB();

    What do you mean by unique names? How to create them? Do you mean creation through "new"?

    0
  • Lifetime Qt Champion

    @Mikeeeeee said in How to connect to the database correctly in QtWebApp:

    What do you mean by unique names?

    RTM: https://doc.qt.io/qt-5/qsqldatabase.html#details

    1

  • I did this from the very beginning and with this approach the server crashes when 2 different objects are connected to the database in parallel

    0
  • Lifetime Qt Champion

    What exactly did you do ?
    What did the stack trace of the crash tell you ?

    0

  • Each new object of the DataBase class reconnects to the database. This object is created each time in a function

    void RequestHandler::service(HttpRequest& request, HttpResponse& response)

    And I get error:

    07.06.2020 15:36:44.134 1 WARNING  0x20d4 QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.
    0 1 Reply
  • Lifetime Qt Champion

    @Mikeeeeee said in How to connect to the database correctly in QtWebApp:

    duplicate connection name 'qt_sql_default_connection'

    It doesn't look like you really read the documentation: you do NOT set database name!
    Also it would be helpful if you would post your code...

    1

  • It is my code fore open database:

    void DataBase::openDB()
{
    QTextStream out(stdout);
    ///*
    dataBase = QSqlDatabase::addDatabase("QPSQL");
    dataBase.setDatabaseName("db_game_tamada");
    dataBase.setUserName("postgres");
    dataBase.setPassword("1qaz");
    dataBase.setPort(5433);//*/
    if (dataBase.open()) {
        //out<<"db is opened \r\n";
    }
    else {
        //out<<"db not opened \r\n";
    }
}

void DataBase::closeDB()
{
    dataBase.close();
}

    I tried to use it like this:

    void RequestHandler::service(HttpRequest& request, HttpResponse& response)
{
    DataBase db;
    db.openDB();
    0
  • Lifetime Qt Champion

    As we already suggested several times: go read the documentation of addDatabase. You are not setting a connection name, therefore all your DataBase objects are using default connection.

    And as I already have written numerous times: Do not keep a local QSqlDatabase member variable. There's a warning in the documentation about that.

    1 1 Reply

  • How I can set connection name? I try this, but this code not work

        dataBase = QSqlDatabase::addDatabase("QPSQL", "conectionName");
    dataBase.setDatabaseName("db_game_tamada");
    dataBase.setUserName("postgres");
    dataBase.setPassword("1qaz");
    dataBase.setPort(5433);//*/
    if (dataBase.open()) {
        //out<<"db is opened \r\n";
    }
    else {
        //out<<"db not opened \r\n";
    }

    I get error: driver not loaded

    0
  • Lifetime Qt Champion

    You need to use a unique name for each connection.

    As for the error, there's usually a bit more information than that and if not, then set the QT_DEBUG_PLUGINS environment variable to 1 to get more output.

    You can do it in the Run part of the Project panel in Qt Creator.

    1

  • But how to set the name of the database connection? My previous version does not work even with 1 connection. Please write a correct example.

    0
  • Lifetime Qt Champion

    You used the way the documentation explains.
    You have now what seems to be an unrelated error.
    You were asked to provide additional information.
    Provide them so we can find what is going on.

    1

  • @SGaist said in How to connect to the database correctly in QtWebApp:

    QT_DEBUG_PLUGINS

    I use windows7 and in cmd I write "QT_DEBUG_PLUGINS=1" and get error: QT_DEBUG_PLUGINS is not an internal or external command.
    How use QT_DEBUG_PLUGINS in windows?

    0 1 Reply

  • @Mikeeeeee
    We have previously told you: under Windows in a Command Prompt you have to write:

    set QT_DEBUG_PLUGINS=1

    If you are developer it's a really basic thing, you should take your own time to read how to set an environment variable, it's not a Qt thing.

    1

  • I get this in server log:

    08.06.2020 13:53:10.942 0 DEBUG    0x26c0 ServiceHelper: Starting service
08.06.2020 13:53:11.042 0 DEBUG    0x26c0 HttpListener: Listening on port 8080
08.06.2020 13:53:11.042 1 WARNING  0x26c0 Startup: Service has started
08.06.2020 13:53:19.333 0 DEBUG    0x26c0 HttpListener: New connection
08.06.2020 13:53:19.333 0 DEBUG    0x26c0 HttpConnectionHandler (0x112c0a0): thread started
08.06.2020 13:53:19.333 0 DEBUG    0x26c0 HttpConnectionHandler (0x112c0a0): constructed
08.06.2020 13:53:19.333 0 DEBUG    0x2838 HttpConnectionHandler (0x112c0a0): handle new connection
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpConnectionHandler (0x112c0a0): read input
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: read request
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: from ::ffff:127.0.0.1: POST /getAllCategory HTTP/1.1
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: received header host: 127.0.0.1:8080
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: received header content-type: application/json
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: received header content-length: 46
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: received header connection: Keep-Alive
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: received header accept-encoding: gzip, deflate
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: received header accept-language: ru-RU,en,*
08.06.2020 13:53:19.334 0 DEBUG    0x2838 HttpRequest: received header user-agent: Mozilla/5.0
08.06.2020 13:53:19.335 0 DEBUG    0x2838 HttpRequest: headers completed
08.06.2020 13:53:19.335 0 DEBUG    0x2838 HttpRequest: expect 46 bytes body
08.06.2020 13:53:19.443 0 DEBUG    0x2838 HttpConnectionHandler (0x112c0a0): read input
08.06.2020 13:53:19.443 0 DEBUG    0x2838 HttpRequest: receive body
08.06.2020 13:53:19.443 0 DEBUG    0x2838 HttpRequest: extract and decode request parameters
08.06.2020 13:53:19.443 0 DEBUG    0x2838 HttpRequest: extract cookies
08.06.2020 13:53:19.443 0 DEBUG    0x2838 HttpConnectionHandler (0x112c0a0): received request
08.06.2020 13:53:19.444 0 DEBUG    0x2838 QFactoryLoader::QFactoryLoader() checking directory path "E:/Programs/Qt/5.15.0/mingw81_64/plugins/sqldrivers" ...
08.06.2020 13:53:19.445 0 DEBUG    0x2838 QFactoryLoader::QFactoryLoader() looking at "E:/Programs/Qt/5.15.0/mingw81_64/plugins/sqldrivers/qsqlite.dll"
08.06.2020 13:53:19.447 1 WARNING  0x2838 Found metadata in lib E:/Programs/Qt/5.15.0/mingw81_64/plugins/sqldrivers/qsqlite.dll, metadata=
{
    "IID": "org.qt-project.Qt.QSqlDriverFactoryInterface",
    "MetaData": {
        "Keys": [
            "QSQLITE"
        ]
    },
    "archreq": 0,
    "className": "QSQLiteDriverPlugin",
    "debug": false,
    "version": 331520
}


08.06.2020 13:53:19.447 0 DEBUG    0x2838 Got keys from plugin meta data ("QSQLITE")
08.06.2020 13:53:19.447 0 DEBUG    0x2838 QFactoryLoader::QFactoryLoader() looking at "E:/Programs/Qt/5.15.0/mingw81_64/plugins/sqldrivers/qsqlodbc.dll"
08.06.2020 13:53:19.448 1 WARNING  0x2838 Found metadata in lib E:/Programs/Qt/5.15.0/mingw81_64/plugins/sqldrivers/qsqlodbc.dll, metadata=
{
    "IID": "org.qt-project.Qt.QSqlDriverFactoryInterface",
    "MetaData": {
        "Keys": [
            "QODBC3",
            "QODBC"
        ]
    },
    "archreq": 0,
    "className": "QODBCDriverPlugin",
    "debug": false,
    "version": 331520
}


08.06.2020 13:53:19.448 0 DEBUG    0x2838 Got keys from plugin meta data ("QODBC3", "QODBC")
08.06.2020 13:53:19.448 0 DEBUG    0x2838 QFactoryLoader::QFactoryLoader() looking at "E:/Programs/Qt/5.15.0/mingw81_64/plugins/sqldrivers/qsqlpsql.dll"
08.06.2020 13:53:19.449 1 WARNING  0x2838 Found metadata in lib E:/Programs/Qt/5.15.0/mingw81_64/plugins/sqldrivers/qsqlpsql.dll, metadata=
{
    "IID": "org.qt-project.Qt.QSqlDriverFactoryInterface",
    "MetaData": {
        "Keys": [
            "QPSQL7",
            "QPSQL"
        ]
    },
    "archreq": 0,
    "className": "QPSQLDriverPlugin",
    "debug": false,
    "version": 331520
}


08.06.2020 13:53:19.449 0 DEBUG    0x2838 Got keys from plugin meta data ("QPSQL7", "QPSQL")
08.06.2020 13:53:19.449 0 DEBUG    0x2838 QFactoryLoader::QFactoryLoader() checking directory path "E:/QTProject/build-HttpSerwerQtWebAppGameTamada-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/debug/sqldrivers" ...
08.06.2020 13:53:19.457 0 DEBUG    0x2838 loaded library "E:/Programs/Qt/5.15.0/mingw81_64/plugins/sqldrivers/qsqlpsql.dll"
08.06.2020 13:53:19.504 1 WARNING  0x2838 QSqlQuery::prepare: database not open
08.06.2020 13:53:19.504 0 DEBUG    0x2838 Conroller: finished request
08.06.2020 13:53:19.504 0 DEBUG    0x2838 HttpConnectionHandler (0x112c0a0): finished request
    0 1 Reply

  • @Mikeeeeee said in How to connect to the database correctly in QtWebApp:

    08.06.2020 13:53:19.504 1 WARNING 0x2838 QSqlQuery::prepare: database not open

    0

  • Yes, if I set connect name, then database not opened. How I can set correctly connect name?

    0
  • Lifetime Qt Champion

    Rather than doing nothing in your if code that checks whether the database is opened, what about printing the error you get form QSqlDatabase ?

    1

  • Sanks, it is work

        dataBase = QSqlDatabase::addDatabase("QPSQL", "conectionName");
    dataBase.setDatabaseName("db_game_tamada");
    dataBase.setUserName("postgres");
    dataBase.setPassword("1qaz");
    //ResumeDB.setHostName("127.0.0.1");
    dataBase.setPort(5433);//*/

    and in query need set database

    QSqlQuery query(dataBase);
    0
  • Lifetime Qt Champion

    Repeating myself:

    @SGaist said in How to connect to the database correctly in QtWebApp:

    You need to use a unique name for each connection.

    @SGaist said in How to connect to the database correctly in QtWebApp:

    And as I already have written numerous times: Do not keep a local QSqlDatabase member variable. There's a warning in the documentation about that.

    0
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