QSqlRelationalTableModel and complex queries

  • I was using QSqlRelationalTableModel for making the model of a QTableView.
    But I had to had some complex queries (LEFT JOINS, plus concatenating fields) (can I concatenate fields with QSqlRelationalTableModel? I think not).
    But, QSqlRelationalTableModel cannot accept queries (the ->setQuery is private), only the ->setTable() works.
    So I replaced QSqlRelationalTableModel with QSqlQueryModel.

    Is this the best approach or am I missing something?

  • The huge difference is that one is editable and the other isn't. If you just need to display data then QSqlQueryModel should work for you, if you need to edit the model and changes be reflected in the database you'll have to implement your own model or use a combination of QStandardItemModel and QSqlQuery

  • @VRonin said in QSqlRelationalTableModel and complex queries:

    if you need to edit the model and changes be reflected in the database you'll have to implement your own model

    I 'm thinking of implementing my own model, but I 'll need some help.
    It's the QSqlTableModel I 'ii have to subclass, right?
    EDIT: abandoned, too complicated, I 'll just use QSqlQueryModel and QSqlQuery. (about QStandardItemModel: I read some thing, but didn't understand much).

  • @Panoss said in QSqlRelationalTableModel and complex queries:

    It's the QSqlTableModel I 'ii have to subclass

    I'd start from scratch with a QAbstractItemModel

    about QStandardItemModel: I read some thing, but didn't understand much

    a good starting point is http://doc.qt.io/qt-5/modelview.html

  • Thanks VRonin, but as I mentioned, I abandon the subclassing.

  • I changed my mind again because I found the code of QSqlTableModel.
    So I 'll go subclassing QSqlTableModel.
    I 'll just add a new function that will set a query.
    What do you think?

  • This post is deleted!

  • How about QAbstractListModel instead of QAbstractItemModel?
    Because I have a list (a QTableView), I suppose this is the one I should use.
    My header:

    #ifndef PGSSQLMODEL_H
    #define PGSSQLMODEL_H
    #include <QAbstractListModel>
    #include <QStringList>
    class PGsSqlModel : public QAbstractListModel
        explicit PGsSqlModel(QObject *parent = 0);
        PGsSqlModel(const QStringList &strings, QObject *parent = 0)
                 : QAbstractListModel(parent), stringList(strings) {}
         int rowCount(const QModelIndex &parent = QModelIndex()) const;
         QVariant data(const QModelIndex &index, int role) const;
         QVariant headerData(int section, Qt::Orientation orientation,
                             int role = Qt::DisplayRole) const;
        QStringList stringList;
    public slots:
    #endif // PGSSQLMODEL_H

    My Source:

    #include "pgssqlmodel.h"
    PGsSqlModel::PGsSqlModel(QObject *parent) : QAbstractListModel(parent)
        stringList << "bbb" << "aaa";
    int PGsSqlModel::rowCount(const QModelIndex &parent) const
        return stringList.count();
    QVariant PGsSqlModel::data(const QModelIndex &index, int role) const
        if (!index.isValid())
            return QVariant();
        if (index.row() >= stringList.size())
            return QVariant();
        if (role == Qt::DisplayRole)
            return stringList.at(index.row());
            return QVariant();
    QVariant PGsSqlModel::headerData(int section, Qt::Orientation orientation, int role) const
        if (role != Qt::DisplayRole)
            return QVariant();
        if (orientation == Qt::Horizontal)
            return QString("Column %1").arg(section);
            return QString("Row %1").arg(section);

    I use set model for a QTableView, but I get a table with two empty rows.
    I should get two rows containing:

    What ust I do?
    And how can I access the database?

  • Lifetime Qt Champion


    How are you setting that model ?

  • Hi SGaist.
    It's ok now, I'm getting the expected result. I was just hiding a column (forgotten code) without noticing it (ok, very silly of me :)).

    I use this to set the model(if this is what you mean):

    PGsSqlModel *manufacturers_tbl_model = new PGsSqlModel;

    How can I access the database from within the class?

  • Lifetime Qt Champion

    Through QSqlQuery

  • I ended up using QAbstractTableModel, I think this is the one I need.
    Now, let 's say I make a function getDataFromDB, and result set is stored in a QStringList (?? I don't know, I 'm asking, maybe you can suggest me what to use).

    Then in ::data function, how shall I use this variable to return the data to the "user"? (I mean the QTableView that will use the data to display them)

  • I'd suggest a QHash<quint64,QVariant> where the key has the most significant 32 bits as the row index and the least significant 32 as the column

  • I 've made this function: (I call it of course, like this: model->setQuery("SELECT id, name FROM parts"))

     void TableModel::setQuery(const QSqlQuery &qry) {
         query = qry;

    I suppose this is not enough for the model to get the data from db and then be available (I mean the data from db) to the model? (a QAbstractTableModel)
    Must I do something more?

    And this is the data function:

     QVariant TableModel::data(const QModelIndex &index, int role) const
         if (!index.isValid())
             return QVariant();
         if (index.row() >= query.size())
             return QVariant();
         if (role == Qt::DisplayRole) {
             return query.value(index.column()); 
         } else {
             return QVariant();

    Where I get an error:
    ...\tablemodel.cpp:74: error: C2662: 'QSqlQuery::seek' : cannot convert 'this' pointer from 'const QSqlQuery' to 'QSqlQuery &'
    Conversion loses qualifiers

    It's at the line 'query.seek(index.row());'.

  • Lifetime Qt Champion

    How did you declared your query member variable ?

  • private:

     QSqlQuery query;

    I read from this:
    "get rid of the const with const_cast"

    What does this mean?

  • "get rid of the const with const_cast"

    mutable QSqlQuery query; solves your compilation issue but I don't think your logic is going anywhere.

    Just a second here. let's take a step back: what do you want to achieve?

    • does the model need to be editable and changes be reflected in the database?
    • will you have more than 1 column?

  • I want my QTableView to display data from two tables.
    This in SQL means my model should have the ability to:

    1. make LEFT JOINS on the second table
    2. Concatenate two fields.

    For these things, QSqlQueryModel works fine.

    But I also need to add and remove records, which QSqlQueryModel cannot do.

    So, my conclusion is I should subclass QSqlQueryModel (this is what I'm doing right now, but I have problems...) and add custom functionality for adding and removing rows.

    What do you think?

  • Personally I would not subclass anything, I'd use a QStandardItemModel fill it with a QSqlQuery and react to QStandardItemModel::rowsInserted and QStandardItemModel::rowsRemoved signals to trigger the update in the db via another QSqlQuery

  • The problem is I don't have the slightest idea how to do all this.
    Is there some example I could look at?

    EDIT: QStandardItemModel has no setQuery function.
    How shall I "fill it with a QSqlQuery"?
    You mean wit QStandardItemModel::setItem?

  • @Panoss said in QSqlRelationalTableModel and complex queries:

    How shall I "fill it with a QSqlQuery"

    manually, outside the model

        model->removeRows(0, model->rowCount());
        model->removeColumns(0, model->columnCount());
        QSqlQuery testQuery;
        testQuery.prepare("select * from MyTable");
        if (testQuery.exec()) {
            for (bool firstRun = true; testQuery.next();) {
                const QSqlRecord currRecord = testQuery.record();
                if (firstRun) {
                    firstRun = false;
                    model->insertColumns(0, currRecord.count());
                    for (int i = 0; i < currRecord.count(); ++i)
                        model->setHeaderData(i, Qt::Horizontal, currRecord.fieldName(i));
                const int newRow = model->rowCount();
                for (int i = 0; i < currRecord.count(); ++i)
                    model->setData(model->index(newRow, i), currRecord.value(i));

  • Works, the model->removeRow works, it removes a row from the model ,but how do I remove this row from the db too? Something like this?

    QSqlQuery query;
        query.prepare("DELETE FROM MyTable WHERE id=1");
        if (query.exec()) {
    etc. etc.

  • Lifetime Qt Champion

    You'll likely have to make a more precise delete query but otherwise, yes.

  • What do you mean by "more precise"? I thought "WHERE id=1" makes it absolutely precise (the value "1" was chosen for the shake of the example).

  • Lifetime Qt Champion

    Sorry, that was badly written, I meant that you would have to ensure that you are passing the correct parameter(s) to the delete query to ensure you are deleting the same row in the database.

  • Ah, ok, guys, VRonin, SGaist, thank you very much.

  • be careful here as if you naively connect the database delete to rowsRemoved the first two lines:
    (model->removeRows(0, model->rowCount()); model->removeColumns(0, model->columnCount());)
    will delete your entire database and you cannot just use a QSignalBlocker on the model either or the view won't update

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