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Cannot connect to databases, Driver not loaded error.

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  • SGaistS Offline
    SGaistS Offline
    SGaist
    Lifetime Qt Champion
    wrote on last edited by
    #3

    Hi and welcome to devnet,

    To add to @Christian-Ehrlicher, this question has been asked many times already on this forum, please take the time to search through it.

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    Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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    3
    • M Offline
      M Offline
      mleoni
      wrote on last edited by
      #4

      Hi Christian, SGaist, thank you for your answers. As I mentioned in my post, I have already searched thoroughly the net for a solution, I have already re-compiled libqsqlmysql.so and calling ldd on it shows that not only does it not miss any dependency, it also links to the same libraries as the homonymous file from Debian's repositories. In particular, it links to the installed version of libmariadbclient [Debian switched to MariaDB which is a fork of MySQL and a drop-in replacement].

      In any case, I also mentioned in my previous post that the problem is not specific to the MySQL driver, but I get the same error even if I try to use SQLite as a backend.

      What is the next thing that I can try?

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      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #5

        Then start your application with the QT_DEBUG_PLUGINS environment variable set to 1. That will give you some more information about what is happening.

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        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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        • M Offline
          M Offline
          mleoni
          wrote on last edited by
          #6

          Thanks for the suggestion!
          This is the relevant output

          QFactoryLoader::QFactoryLoader() checking directory path "/opt/Qt/5.10.1/gcc_64/plugins/sqldrivers" ...
          QFactoryLoader::QFactoryLoader() looking at "/opt/Qt/5.10.1/gcc_64/plugins/sqldrivers/libqsqlite.so"
          Found metadata in lib /opt/Qt/5.10.1/gcc_64/plugins/sqldrivers/libqsqlite.so, metadata=
          {
              "IID": "org.qt-project.Qt.QSqlDriverFactoryInterface",
              "MetaData": {
                  "Keys": [
                      "QSQLITE"
                  ]
              },
              "className": "QSQLiteDriverPlugin",
              "debug": false,
              "version": 330241
          }
          
          
          Got keys from plugin meta data ("QSQLITE")
          QFactoryLoader::QFactoryLoader() looking at "/opt/Qt/5.10.1/gcc_64/plugins/sqldrivers/libqsqlmysql.so"
          Found metadata in lib /opt/Qt/5.10.1/gcc_64/plugins/sqldrivers/libqsqlmysql.so, metadata=
          {
              "IID": "org.qt-project.Qt.QSqlDriverFactoryInterface",
              "MetaData": {
                  "Keys": [
                      "QMYSQL3",
                      "QMYSQL"
                  ]
              },
              "className": "QMYSQLDriverPlugin",
              "debug": false,
              "version": 330241
          }
          
          
          Got keys from plugin meta data ("QMYSQL3", "QMYSQL")
          
          
          loaded library "/opt/Qt/5.10.1/gcc_64/plugins/sqldrivers/libqsqlmysql.so"
          "Driver not loaded Driver not loaded"
          

          Meanwhile I have run the Qt example sqlbrowser and it does successfully connect to my MySQL local server, it shows me the database and the tables. This makes me suspect that there is an error in the code, but what could it be?

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          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #7

            Debug VS Release version of the plugin ?

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            Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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            0
            • M Offline
              M Offline
              mleoni
              wrote on last edited by
              #8

              Sorry, I don't understand the question :(
              Are you asking me to compare two different builds of the same plugin?

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              0
              • SGaistS Offline
                SGaistS Offline
                SGaist
                Lifetime Qt Champion
                wrote on last edited by
                #9

                Did you build your plugin for both debug and release ?

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                Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                M 1 Reply Last reply
                0
                • H Offline
                  H Offline
                  Homer2000
                  wrote on last edited by
                  #10

                  need dynamic lib for mysql. For linux it's libmysqlclient.so.16 or libmysqlclient.so.18 for your target platform.

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                  • SGaistS SGaist

                    Did you build your plugin for both debug and release ?

                    M Offline
                    M Offline
                    mleoni
                    wrote on last edited by
                    #11

                    @SGaist I built just one, with the default configuration. I will check which one when I get back to my home pc. What difference does it make? Could it work with one but not the other one?

                    @Homer2000 I am using a shared library, except my distribution [like many others] now uses MariaDB as a replacement for MySQL, so I link to libmariadb.so or something similar

                    H 1 Reply Last reply
                    0
                    • M mleoni

                      @SGaist I built just one, with the default configuration. I will check which one when I get back to my home pc. What difference does it make? Could it work with one but not the other one?

                      @Homer2000 I am using a shared library, except my distribution [like many others] now uses MariaDB as a replacement for MySQL, so I link to libmariadb.so or something similar

                      H Offline
                      H Offline
                      Homer2000
                      wrote on last edited by
                      #12

                      @mleoni said in Cannot connect to databases, Driver not loaded error.:

                      MariaDB

                      MariaDB use libqsqlmysql plugin?

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                      • SGaistS Offline
                        SGaistS Offline
                        SGaist
                        Lifetime Qt Champion
                        wrote on last edited by
                        #13

                        @Homer2000 No, by default the Qt MySQL plugin links against the MySQL client libraries.

                        You can build it to use MariaDB though.

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                        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                        • M Offline
                          M Offline
                          mleoni
                          wrote on last edited by
                          #14

                          Ok, I found out what I was doing wrong.
                          The code that I was using is

                          QSqlDatabase db;
                          db.addDatabase("QMYSQL");
                          

                          and it was giving me the error above, whereas the correct way to do it is

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

                          and with this code it works fine.

                          I don't know why this is the case, I guess today I just learnt something new on Qt, thanks for the help everyboody!

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                          • Christian EhrlicherC Offline
                            Christian EhrlicherC Offline
                            Christian Ehrlicher
                            Lifetime Qt Champion
                            wrote on last edited by
                            #15

                            @mleoni said in Cannot connect to databases, Driver not loaded error.:

                            I don't know why this is the case,

                            Because QSqlDatabase::addDatabase() is a static function - no Qt magic here.

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                            • M Offline
                              M Offline
                              mleoni
                              wrote on last edited by
                              #16

                              I see, so my mistake was letting the returned value go lost. Thanks for clarifying this!

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                              0
                              • SGaistS Offline
                                SGaistS Offline
                                SGaist
                                Lifetime Qt Champion
                                wrote on last edited by
                                #17

                                Please take the time to read the QSqlDatabase documentation. It shows how to use it properly.

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

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                                • M Offline
                                  M Offline
                                  mleoni
                                  wrote on last edited by
                                  #18

                                  Thanks for the reference!

                                  1 Reply Last reply
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