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Thread: QsqlQueryModel Subclass

  1. 13th May 2013, 09:37 #1
    Delphi
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    Default QsqlQueryModel Subclass

    This question may have been answered somewhere else already, but I just can't seem to find the answer. I'm using the model to display a set of data from a sql database. I'm using the QSqlQueryModel to retrieve the data.

    In the QSqlQueryModel I need to add a column that is editable for the user. The data needs to be saved in the model and not committed to the database. This means that the setData function from QSqlQueryModel needs to be implemented and change the internal data. Is this possible? And is there an example?

    Any help is appreciated, thanks.
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  2. 13th May 2013, 11:08 #2
    Santosh Reddy's Avatar
    Santosh Reddy
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    Default Re: QsqlQueryModel Subclass

    The better way is make use of a custom proxy model, and not disturb the QSqlQueryModel. Moreover implementing QSqlQueryModel::setData() may not be practical as you don't know how the QSqlQueryModel stores the data internally.

    The proxy model will add the extra column.
    When you know how to do it then you may do it wrong.
    When you don't know how to do it then it is not that you may do it wrong but you may not do it right.
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    Delphi (13th May 2013)
  4. 13th May 2013, 14:07 #3
    Delphi
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    Default Re: QsqlQueryModel Subclass

    Thanks for your replay,

    So one of the correct ways, would be to create a QAbstractProxyModel subclass with the QSqlQuery model as sourcemodel.

    which functions do i need to implement/reimplement? by for of them i have no idea.

    Any help is appreciated, thanks.

    Qt Code: 
    1. class SqlProxyModelAddColumn : public QAbstractProxyModel
    2. {
    3. public:
    4.     SqlProxyModelAddColumn() : QAbstractProxyModel(){
    5.  
    6.     }
    7.  
    8.     virtual ~SqlProxyModelAddColumn(){
    9.  
    10.     }
    11.  
    12.     virtual QModelIndex mapFromSource( const QModelIndex & sourceIndex) const{
    13.  
    14.         // what to do here?
    15.     }
    16.     virtual QModelIndex mapToSource( const QModelIndex & proxyIndex) const{
    17.  
    18.        // what to do here?
    19.     }
    20.  
    21.     virtual QModelIndex index(int row, int column, const QModelIndex &parent = QModelIndex()) const{
    22.  
    23.         // what to do here?
    24.  
    25.         if (row < 0 || column < 0)
    26.                return QModelIndex();
    27.  
    28.         return createIndex(row, column); 
    29.     }
    30.     virtual QModelIndex parent(const QModelIndex &child) const{
    31.  
    32.         // what to do here?
    33.     }
    34.  
    35.     virtual Qt::ItemFlags flags ( const QModelIndex & index ) const{
    36.  
    37.         if (index.column() == sourceModel()->columnCount()) // extra colum
    38.             return Qt::ItemIsEditable | Qt::ItemIsSelectable;
    39.         return sourceModel()->flags(index);
    40.  
    41.     }
    42.  
    43.     virtual bool setData( const QModelIndex & index, const QVariant & value, int role = Qt::EditRole ){
    44.  
    45.         if (role != Qt::EditRole)
    46.             return false;
    47.  
    48.         if (!index.isValid())
    49.             return false;
    50.  
    51.         if (index.column() == sourceModel()->columnCount()){ // extra column
    52.  
    53.             for ( int i = 0; i < LastColumn.count();++i){
    54.  
    55.                 if (LastColumn[i].first == index.row()){
    56.                     LastColumn.removeAt(i);
    57.                     LastColumn.append(qMakePair(index.row(),value.toString()));
    58.                     return true;
    59.                 }
    60.  
    61.                LastColumn.append(qMakePair(index.row(),value.toString()));
    62.                return true;
    63.             }
    64.         }
    65.  
    66.         return sourceModel()->setData(index, value, role);
    67.     }
    68.  
    69.     virtual QVariant data ( const QModelIndex & proxyIndex, int role = Qt::DisplayRole ) const{
    70.  
    71.         if (role != Qt::DisplayRole)
    72.             return false;
    73.  
    74.         if (!proxyIndex.isValid())
    75.             return false;
    76.  
    77.         if (proxyIndex.column() == sourceModel()->columnCount()){ // extra column
    78.  
    79.             for ( int i = 0; i < LastColumn.count();++i){
    80.  
    81.                 if (LastColumn[i].first == proxyIndex.row())
    82.                     return LastColumn[i].second;
    83.             }
    84.  
    85.             return QString();
    86.         }
    87.  
    88.         return sourceModel()->data(proxyIndex, role);
    89.     }
    90.  
    91.     virtual int rowCount(const QModelIndex &parent = QModelIndex()) const{
    92.  
    93.         return sourceModel->rowCount()+1;
    94.     }
    95.  
    96.     virtual int columnCount(const QModelIndex &parent = QModelIndex()) const{
    97.  
    98.         return sourceModel->columnCount()+1;
    99.     }
    100.  
    101. private:
    102.     QList<QPair<int,QString> > LastColumn;
    103.  
    104. };
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  5. 13th May 2013, 16:45 #4
    Santosh Reddy's Avatar
    Santosh Reddy
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    Default Re: QsqlQueryModel Subclass

    Here is an example
    ProxyModel.jpg
    Qt Code: 
    1. #include <QtGui>
    2. #include <QtSql>
    3. #include <QApplication>
    4.  
    5. // Add one extra column at the beginning
    6. class ProxyModel : public QAbstractProxyModel
    7. {
    8.     Q_OBJECT
    9. public:
    10.     explicit ProxyModel(QObject * parent = 0)
    12.     {
    13.         ;
    14.     }
    15.  
    16.     void setSourceModel(QAbstractItemModel *sourceModel)
    17.     {
    18.         if(this->sourceModel() != sourceModel)
    19.         {
    20.             if(this->sourceModel() != 0)
    21.             {
    22.                 disconnect(this->sourceModel(), SIGNAL(rowsAboutToBeInserted(const QModelIndex &, int, int)),
    23.                            this, SIGNAL(rowsAboutToBeInserted(const QModelIndex &, int, int)));
    24.                 disconnect(this->sourceModel(), SIGNAL(rowsInserted(const QModelIndex &, int, int)),
    25.                            this, SIGNAL(rowsInserted(const QModelIndex &, int, int)));
    26.                 disconnect(this->sourceModel(), SIGNAL(columnsAboutToBeInserted(const QModelIndex &, int, int)),
    27.                            this, SIGNAL(columnsAboutToBeInserted (const QModelIndex &, int, int)));
    28.                 disconnect(this->sourceModel(), SIGNAL(columnsInserted(const QModelIndex &, int, int)),
    29.                            this, SIGNAL(columnsInserted(const QModelIndex &, int, int)));
    30.                 disconnect(this->sourceModel(), SIGNAL(modelAboutToBeReset()),
    31.                            this, SIGNAL(modelAboutToBeReset()));
    32.                 disconnect(this->sourceModel(), SIGNAL(modelReset()),
    33.                            this, SIGNAL(modelReset()));
    34.                 disconnect(this->sourceModel(), SIGNAL(layoutAboutToBeChanged()),
    35.                            this, SIGNAL(layoutAboutToBeChanged()));
    36.                 disconnect(this->sourceModel(), SIGNAL(layoutChanged()),
    37.                            this, SIGNAL(layoutChanged()));
    38.                 disconnect(this->sourceModel(), SIGNAL(dataChanged(const QModelIndex &, const QModelIndex &)),
    39.                            this, SIGNAL(dataChanged(const QModelIndex &, const QModelIndex &)));
    40.             }
    41.  
    42.             QAbstractProxyModel::setSourceModel(sourceModel);
    43.  
    44.             connect(this->sourceModel(), SIGNAL(rowsAboutToBeInserted(const QModelIndex &, int, int)),
    45.                     this, SIGNAL(rowsAboutToBeInserted(const QModelIndex &, int, int)));
    46.             connect(this->sourceModel(), SIGNAL(rowsInserted(const QModelIndex &, int, int)),
    47.                     this, SIGNAL(rowsInserted(const QModelIndex &, int, int)));
    48.             connect(this->sourceModel(), SIGNAL(columnsAboutToBeInserted (const QModelIndex &, int, int)),
    49.                     this, SIGNAL(columnsAboutToBeInserted(const QModelIndex &, int, int)));
    50.             connect(this->sourceModel(), SIGNAL(columnsInserted(const QModelIndex &, int, int)),
    51.                     this, SIGNAL(columnsInserted(const QModelIndex &, int, int)));
    52.             connect(this->sourceModel(), SIGNAL(modelAboutToBeReset()),
    53.                     this, SIGNAL(modelAboutToBeReset()));
    54.             connect(this->sourceModel(), SIGNAL(modelReset()),
    55.                     this, SIGNAL(modelReset()));
    56.             connect(this->sourceModel(), SIGNAL(modelReset()),
    57.                     this, SIGNAL(modelReset()));
    58.             connect(this->sourceModel(), SIGNAL(layoutAboutToBeChanged()),
    59.                     this, SIGNAL(layoutAboutToBeChanged()));
    60.             connect(this->sourceModel(), SIGNAL(layoutChanged()),
    61.                     this, SIGNAL(layoutChanged()));
    62.             connect(this->sourceModel(), SIGNAL(dataChanged(const QModelIndex &, const QModelIndex &)),
    63.                     this, SIGNAL(dataChanged(const QModelIndex &, const QModelIndex &)));
    64.         }
    65.     }
    66.  
    67.     QModelIndex index(int row, int column, const QModelIndex & parent = QModelIndex()) const
    68.     {
    69.         if(!parent.isValid())
    70.         {
    71.             if(column == 0)
    72.                 return createIndex(row, column, -1);
    73.             else
    74.                 return createIndex(row, column, 0);
    75.         }
    76.         return QModelIndex();
    77.     }
    78.  
    79.     QModelIndex parent(const QModelIndex & /*child*/) const
    80.     {
    81.         return QModelIndex();
    82.     }
    83.  
    84.     int rowCount(const QModelIndex & parent = QModelIndex()) const
    85.     {
    86.         if(!parent.isValid())
    87.             return sourceModel()->rowCount(parent);
    88.         return 0;
    89.     }
    90.  
    91.     int columnCount(const QModelIndex & parent = QModelIndex()) const
    92.     {
    93.         if(!parent.isValid())
    94.             return sourceModel()->columnCount(parent) + 1;
    95.         return 0;
    96.     }
    97.  
    98.     QModelIndex mapToSource(const QModelIndex & proxyIndex) const
    99.     {
    100.         if(proxyIndex.column() <= 0)
    101.             return QModelIndex();
    102.  
    103.         return sourceModel()->index(proxyIndex.row(), proxyIndex.column() - 1);
    104.     }
    105.  
    106.     QModelIndex mapFromSource(const QModelIndex & sourceIndex) const
    107.     {
    108.         return index(sourceIndex.row(), sourceIndex.column() + 1);
    109.     }
    110.  
    111.     QVariant headerData(int section, Qt::Orientation orientation, int role = Qt::DisplayRole) const
    112.     {
    113.         if(orientation == Qt::Horizontal)
    114.         {
    115.             if(section <= 0)
    116.                 return QString("Proxy Column");
    117.             return sourceModel()->headerData(section - 1, orientation, role);
    118.         }
    119.         return sourceModel()->headerData(section, orientation, role);
    120.     }
    121.  
    122.     QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const
    123.     {
    124.         if((index.column() <= 0) and (role == Qt::DisplayRole))
    125.             return QString("Proxy Data - Row:%1").arg(index.row());
    126.  
    127.         return sourceModel()->data(mapToSource(index), role);
    128.     }
    129.  
    130.     bool setData(const QModelIndex &index, const QVariant &value, int role = Qt::EditRole)
    131.     {
    132.         if((index.column() <= 0) and (role == Qt::EditRole))
    133.         {
    134.             // TODO: store data into ProxyModel
    135.             return true;
    136.         }
    137.         return sourceModel()->setData(mapToSource(index), role);
    138.     }
    139. };
    140.  
    141. int main(int argc, char *argv[])
    142. {
    143.     QApplication app(argc, argv);
    144.  
    145.     QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    146.     db.setDatabaseName("Qt.sqlite");
    147.  
    149.  
    150.     ProxyModel proxy;
    151.     proxy.setSourceModel(&model);
    152.  
    153.     QTableView view;
    154.     view.setWindowTitle("QSqlQueryModel View");
    155.     view.setModel(&model);
    156.     view.show();
    157.  
    158.     QTableView proxyView;
    159.     proxyView.setWindowTitle("ProxyModel View");
    160.     proxyView.setModel(&proxy);
    161.     proxyView.show();
    162.  
    163.     if(db.open())
    164.         model.setQuery("SELECT Country, Location FROM Locations");
    165.  
    166.     view.resizeColumnsToContents();
    167.     proxyView.resizeColumnsToContents();
    168.  
    169.     return app.exec();
    170. }
    171.  
    172. #include "main.moc"
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    When you know how to do it then you may do it wrong.
    When you don't know how to do it then it is not that you may do it wrong but you may not do it right.
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    Delphi (15th May 2013)
  7. 15th May 2013, 10:37 #5
    Delphi
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    Default Re: QsqlQueryModel Subclass

    Thanks for your replay,

    I implemented data and setdata from your example. But the data in the proxy column is not editable.
    Even when i reimplemented flags the column stayed gray and did not call setdata.
    EDIT sloved graynes by returning Qt::ItemIsEnabled flag EDIT
    What I'm doing wrong?

    compiling the code with qt 5 i got an error on row 74.

    return createIndex(row, column, 0);
    call of overloaded 'createIndex(int&, int&, int)' is ambiguous
    so i used the default. createIndex(int&, int&)


    Qt Code: 
    1. QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const
    2.     {
    3.         if((index.column() <= 0) and (role == Qt::DisplayRole)){
    4.  
    5.            //return QString("Proxy Data - Row:%1").arg(index.row());
    6.  
    7.             for (int i = 0; i < LastColumn.count();++i){
    8.  
    9.                 if (LastColumn[i].first == index.row())
    10.                     return LastColumn[i].second;
    11.             }
    12.  
    13.             return QString("temp");
    14.  
    15.         }
    16.  
    17.         return sourceModel()->data(mapToSource(index), role);
    18.     }
    19.  
    20.     bool setData(const QModelIndex &index, const QVariant &value, int role = Qt::EditRole)
    21.     {
    22.     qDebug() << "setdata";
    23.  
    24.         if((index.column() <= 0) and (role == Qt::EditRole))
    25.         {
    26.             // TODO: store data into ProxyModel
    27.             for ( int i = 0; i < LastColumn.count();++i){
    28.  
    29.                 if (LastColumn[i].first == index.row()){
    30.                     LastColumn.removeAt(i);
    31.                     LastColumn.append(qMakePair(index.row(),value.toString()));
    32.                     return true;
    33.                 }
    34.             }
    35.  
    36.             LastColumn.append(qMakePair(index.row(),value.toString()));
    37.             return true;
    38.         }
    39.         return sourceModel()->setData(mapToSource(index), role);
    40.     }
    41.  
    42.     Qt::ItemFlags flags ( const QModelIndex & index ) const{
    43.  
    44.         if (index.column() <= 0){
    45.  
    46.             return (Qt::ItemIsEditable | Qt::ItemIsSelectable | Qt::ItemIsEnabled);
    47.  
    48.         }
    49.  
    50.         return QAbstractProxyModel::flags(index);
    51.  
    52.     }
    53.  
    54.     private:
    55.         QList<QPair<int,QString> > LastColumn;
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    Last edited by Delphi; 15th May 2013 at 12:44.
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  8. 15th May 2013, 13:45 #6
    Santosh Reddy's Avatar
    Santosh Reddy
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    Thumbs up Re: QsqlQueryModel Subclass

    Ok I have ported to Qt 5.0.2, and the new column is not editable but cannot be saved, hope you can figure that out.

    Basically the problem is with QAbstractProxyModel, which extensively uses mapToSource(), and mapFromSource()
    mapToSource(): All proxy indexes (except the first column) will have a unique source index. All the first column indexes will map to invalid index
    mapFromSource(): All source indexes will have a uniue proxy index. This is just fine, but the first column indexes in proxy model cannot be accesable from any source index. Other way of saying it will be that mapFromSource() function will never be able to return a proxy model index from the first column.

    As a work around for this I created the invalid dummy index to able to get to the first column index of proxy model

    Any why I hope you understand what I did, here is the working code
    Qt Code: 
    1. #include <QtGui>
    2. #include <QtSql>
    3. #include <QtWidgets>
    4. #include <QApplication>
    5.  
    6. // Add one extra column at the begining
    7. class ProxyModel : public QAbstractProxyModel
    8. {
    9.     Q_OBJECT
    10. public:
    11.     explicit ProxyModel(QObject * parent = 0)
    13.     {
    14.         ;
    15.     }
    16.  
    17.     void setSourceModel(QAbstractItemModel *sourceModel)
    18.     {
    19.         if(this->sourceModel() != sourceModel)
    20.         {
    21.             if(this->sourceModel() != 0)
    22.             {
    23.                 disconnect(this->sourceModel(), SIGNAL(modelAboutToBeReset()),
    24.                            this, SIGNAL(modelAboutToBeReset()));
    25.                 disconnect(this->sourceModel(), SIGNAL(modelReset()),
    26.                            this, SIGNAL(modelReset()));
    27.                 disconnect(this->sourceModel(), SIGNAL(layoutAboutToBeChanged()),
    28.                            this, SIGNAL(layoutAboutToBeChanged()));
    29.                 disconnect(this->sourceModel(), SIGNAL(layoutChanged()),
    30.                            this, SIGNAL(layoutChanged()));
    31.             }
    32.  
    33.             QAbstractProxyModel::setSourceModel(sourceModel);
    34.  
    35.             connect(this->sourceModel(), SIGNAL(modelAboutToBeReset()),
    36.                     this, SIGNAL(modelAboutToBeReset()));
    37.             connect(this->sourceModel(), SIGNAL(modelReset()),
    38.                     this, SIGNAL(modelReset()));
    39.             connect(this->sourceModel(), SIGNAL(modelReset()),
    40.                     this, SIGNAL(modelReset()));
    41.             connect(this->sourceModel(), SIGNAL(layoutAboutToBeChanged()),
    42.                     this, SIGNAL(layoutAboutToBeChanged()));
    43.             connect(this->sourceModel(), SIGNAL(layoutChanged()),
    44.                     this, SIGNAL(layoutChanged()));
    45.         }
    46.     }
    47.  
    48.     QModelIndex index(int row, int column, const QModelIndex & parent = QModelIndex()) const
    49.     {
    50.         if(!parent.isValid())
    51.             return createIndex(row, column);
    52.         return QModelIndex();
    53.     }
    54.  
    55.     QModelIndex parent(const QModelIndex & /*child*/) const
    56.     {
    57.         return QModelIndex();
    58.     }
    59.  
    60.     int rowCount(const QModelIndex & parent = QModelIndex()) const
    61.     {
    62.         if(!parent.isValid())
    63.             return sourceModel()->rowCount(QModelIndex());
    64.         return 0;
    65.     }
    66.  
    67.     int columnCount(const QModelIndex & parent = QModelIndex()) const
    68.     {
    69.         if(!parent.isValid())
    70.             return sourceModel()->columnCount() + 1;
    71.         return 0;
    72.     }
    73.  
    74.     QModelIndex mapToSource(const QModelIndex & proxyIndex) const
    75.     {
    76.         if(!proxyIndex.isValid())
    77.             return QModelIndex();
    78.  
    79.         if(proxyIndex.column() == 0)
    80.             return createIndex(proxyIndex.row(), -1, (quintptr)-1); //<<<<<<<<<<<<<<<<<<<<<< This is a workaround. Note is weird to create a index of proxy model and return as if it were an index of sourceModel(), it is not a common but will work (at-least in this case)
    81.  
    82.         return sourceModel()->index(proxyIndex.row(), proxyIndex.column() - 1);
    83.     }
    84.  
    85.     QModelIndex mapFromSource(const QModelIndex & sourceIndex) const
    86.     {
    87.         if(!sourceIndex.isValid())
    88.         {
    89.             if((sourceIndex.row() > -1)                     and //<<<<<<<<<<<<<<<<<<<<<< This is a workaround.
    90.                (sourceIndex.column() == -1)                 and
    91.                (sourceIndex.internalId() == (quintptr)-1)     )
    92.                 return index(sourceIndex.row(), 0);
    93.             else
    94.                 return QModelIndex();
    95.         }
    96.  
    97.         return index(sourceIndex.row(), sourceIndex.column() + 1);
    98.     }
    99.  
    100.     QVariant headerData(int section, Qt::Orientation orientation, int role = Qt::DisplayRole) const
    101.     {
    102.         if(orientation == Qt::Horizontal)
    103.         {
    104.             if(section == 0)
    105.                 return QString("Proxy Column");
    106.             return sourceModel()->headerData(section - 1, orientation, role);
    107.         }
    108.         return sourceModel()->headerData(section, orientation, role);
    109.     }
    110.  
    111.     QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const
    112.     {
    113.         if(index.column() == 0)
    114.         {
    115.             if((role == Qt::DisplayRole) or (role == Qt::EditRole))
    116.                 return QString("Proxy Data - Row:%1").arg(index.row());
    117.             else
    118.                 return QVariant();
    119.         }
    120.  
    121.         return sourceModel()->data(mapToSource(index), role);
    122.     }
    123.  
    124.     bool setData(const QModelIndex & index, const QVariant & value, int role = Qt::EditRole)
    125.     {
    126.         if((index.column() == 0) and
    127.            ((role == Qt::DisplayRole) or (role == Qt::EditRole)))
    128.         {
    129.             // TODO: store data into ProxyModel for only column 0
    130.             (void)value;
    131.             emit dataChanged(index, index);
    132.             return true;
    133.         }
    134.  
    135.         return sourceModel()->setData(mapToSource(index), value, role);
    136.     }
    137.  
    138.     Qt::ItemFlags flags(const QModelIndex & index) const
    139.     {
    140.         if(index.column() == 0)
    141.         {
    142.             Qt::ItemFlags flag = Qt::ItemIsEnabled | Qt::ItemIsSelectable | Qt::ItemIsEditable;
    143.             return flag;
    144.         }
    145.  
    146.         return sourceModel()->flags(mapToSource(index));
    147.     }
    148. };
    149.  
    150. #define USE_QSTANDARDMODEL
    151.  
    152. int main(int argc, char *argv[])
    153. {
    154.     QApplication app(argc, argv);
    155.  
    156. #ifdef USE_QSTANDARDMODEL
    158.     model.setRowCount(5);
    159.     model.setColumnCount(2);
    160. #else // USE_QSTANDARDMODEL
    161.     QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    162.     db.setDatabaseName("Qt.sqlite");
    163.  
    165. #endif // USE_QSTANDARDMODEL
    166.  
    167.     ProxyModel proxy;
    168.     proxy.setSourceModel(&model);
    169.  
    170.     QTableView view;
    171.     view.setWindowTitle("QSqlQueryModel View");
    172.     view.setModel(&model);
    173.     view.show();
    174.  
    175.     QTableView proxyView;
    176.     proxyView.setWindowTitle("ProxyModel View");
    177.     proxyView.setModel(&proxy);
    178.     proxyView.show();
    179.     proxyView.setEditTriggers(QTableView::DoubleClicked);
    180.     proxyView.setItemDelegate(new QStyledItemDelegate(&proxyView));
    181.  
    182. #ifndef USE_QSTANDARDMODEL
    183.     if(db.open())
    184.         model.setQuery("SELECT Country, Location FROM Locations");
    185. #endif // USE_QSTANDARDMODEL
    186.  
    187.     view.resizeColumnsToContents();
    188.     proxyView.resizeColumnsToContents();
    189.  
    190.     return app.exec();
    191. }
    192. #include "main.moc"
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    When you don't know how to do it then it is not that you may do it wrong but you may not do it right.
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    Delphi (15th May 2013)
  10. 15th May 2013, 14:58 #7
    Delphi
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    Default Re: QsqlQueryModel Subclass

    Thanks again,

    it is now clear for me, i thought that mapFromSource was not called by setting data.

    There is only 1 thing i do not understand, modelReset() is connected twice( r 37/39), so this is just a copy-past mistake or is it useful?
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  11. 15th May 2013, 15:07 #8
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    Default Re: QsqlQueryModel Subclass

    There is only 1 thing i do not understand, modelReset() is connected twice( r 37/39), so this is just a copy-past mistake or is it useful?
    Yes it is copy paste leftovers, just ignore that. Moreove all the signal/slot connections in there are not required, just use the ones required for your functionality, also note that you may need to add more connections if required.
    When you know how to do it then you may do it wrong.
    When you don't know how to do it then it is not that you may do it wrong but you may not do it right.
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    Delphi (15th May 2013)
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