How create a calculated column on a QSqlTableModel?

[lqsa]

I would like to show a calculated column on a QTableView where the source is a QSqlTableModel. I've tried a custom QIdentityProxyModel, but the data method for the added column is never called.

For example, if the database has 3 fields, and added a column with the addColumn method, the data method is never called with an index with column 3, only 0, 1 and 2 (the 3 database fields). However, columnCount returns 4 (3 database fields + 1 of the added column).

class CalculatedColumn
{
public:
    CalculatedColumn();
    virtual QVariant calculate(int row) const = 0;
};


class SqlQueryModelCalculatedColumns: public QIdentityProxyModel
{
public:
    SqlQueryModelCalculatedColumns(QObject *parent = 0);
    void addColumn(CalculatedColumn *column);
    int columnCount(const QModelIndex &parent) const override;
    QVariant data(const QModelIndex &index, int role) const override;    
private:
    QVector<CalculatedColumn *> m_columns;
}


SqlQueryModelCalculatedColumns::SqlQueryModelCalculatedColumns(QObject *parent): QIdentityProxyModel(parent)
{
}

void SqlQueryModelCalculatedColumns::addColumn(CalculatedColumn *column)
{
    const int count = sourceModel()->columnCount();
    beginInsertColumns(QModelIndex(), count, count);
    m_columns.append(column);
    endInsertColumns();
}

int SqlQueryModelCalculatedColumns::columnCount(const QModelIndex &parent) const
{
    return sourceModel()->columnCount(parent) + m_columns.count();
}

QVariant SqlQueryModelCalculatedColumns::data(const QModelIndex &index, int role) const
{
    const int column = index.column();
    const int count = sourceModel()->columnCount();
    if (column < count) {
        return sourceModel()->data(index, role);
    }
    else {
        const int iCalculatedColumn = column - count;
        CalculatedColumn *calculatedColumn = m_columns.at(iCalculatedColumn);
        return calculatedColumn->calculate(index.row());
    }
}

[SGaist]

Hi,

Why not subclass QSqlTableModel, reimplement columnCount and do your magic in data ?

[lqsa]

I did it. It was the first attempt, but occurs the same.

[JonB]

@lqsa
If you're saying you did originally take @SGaist 's approach, but it "didn't work", we'll need to see that code....

[lqsa]

Only appears the dataset columns:

class SqlTableModel : public QSqlTableModel {
public:
    explicit SqlTableModel(QObject *parent = 0);    
    QVariant data(const QModelIndex &index, int role) const override;
    int columnCount(const QModelIndex &parent) const override;
};



SqlTableModel::SqlTableModel(QObject *parent): QSqlTableModel(parent)
{
}

QVariant SqlTableModel::data(const QModelIndex &index, int role) const
{
    if ((role == Qt::DisplayRole) && (index.column() == columnCount(QModelIndex()))) {
        return "Hello";
    }
    else {
        return QSqlTableModel::data(index, role);
    }
}

int SqlTableModel::columnCount(const QModelIndex &parent) const
{
    const int count = QSqlTableModel::columnCount(parent) +1;
    return count;
}

[JonB]

@lqsa said in How create a calculated column on a QSqlTableModel?:

index.column() == columnCount(QModelIndex())

How is this right? Index column will never be equal to columnCount()? Don't you mean
index.column() == columnCount(QModelIndex()) - 1
?

I don't know whether that will solve your problem, but it's a start.

[lqsa]

@JonB Yes, must be -1. Now this simple sample works. Thank you very much.