Question

I am trying to get to grips with how you display data from a database into a Qt table. What I want to do is change the displayed value on a boolean to read “PASS” or “FAIL” and then change the row colour depending on this.

I have created an QSqlQueryModel derived class for the model and overrode the function:

QVariant TestResultsViewModel::data(const QModelIndex &index, int role) const
{
    QVariant value = QSqlQueryModel::data(index, role);

    if (value.isValid() && role == Qt::DisplayRole){
        switch(index.column()){
        case 0:
            return value.toBool() ? "PASS" : "FAIL";
        }
    }

    if (role == Qt::TextColorRole){
        // Get column 0
        QVariant pass = index.sibling(index.row(), 0).data();
        if (pass.isValid()){
            if (pass.toBool()){
                return QVariant::fromValue(QColor(Qt::blue));
            }
            else{
                return QVariant::fromValue(QColor(Qt::red));
            }
        }
    }

    return value;
}

But what seems to happen is that the first part is done first and thereafter the values of the column are “PASS” or “FAIL” and not 0, 1 so the colours aren’t changing.

So how should I really be doing this?

Answer 1

The implementation of QModelIndex::data() explains this behaviour:

inline QVariant QModelIndex::data(int arole) const
{ return m ? m->data(*this, arole) : QVariant(); }

where m is the model of the model index.

This means your call of

index.sibling(index.row(), 0).data(); 

ends up calling TestResultsViewModel::data() again, meaning you indeed get “PASS” or “FAIL” as a result.

In order to avoid this, you can do the following:

QModelIndex firstColumnIndex = index.sibling( index.row(), 0 );
QVariant pass = QSqlQueryModel::data( firstColumnIndex );

This will explicitly call QSqlQueryModel::data() instead of TestResultsViewModel::data() and skip your code