QML and QSqlTableModel

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I wrote a simple class derived from QSqlRelationalTableModel to facilitate the use of such classes with QML, without manually defining the roles.

If you want to understand why roles are so important when working with SQL table models and QML, have a look at this explained code snippet

class QLSqlTableModel : public QSqlRelationalTableModel
{
    Q_OBJECT

private:
    QHash<int, QByteArray> roles;

public:
    QLSqlTableModel(QObject *parent = 0);
    ~QLSqlTableModel();

public:
    Q_INVOKABLE QVariant data(const QModelIndex &index, int role=Qt::DisplayRole ) const;
    void generateRoleNames();

#ifdef HAVE_QT5
    virtual QHash<int, QByteArray> roleNames() const{return roles;}
#endif
}

QVariant QLSqlTableModel::data ( const QModelIndex & index, int role ) const
{
    if(index.row() >= rowCount())
    {
        return QString("");
    }
    if(role < Qt::UserRole)
    {
        return QSqlQueryModel::data(index, role);
    } 
    else 
    {
        // search for relationships
        for (int i = 0; i < columnCount(); i+'') 
        {
            if (this->relation(i).isValid()) 
            {
            return record(index.row()).value(QString(roles.value(role)));
            }
        }
    // if no valid relationship was found
    return QSqlQueryModel::data(this->index(index.row(), role - Qt::UserRole - 1), Qt::DisplayRole);
    }
}

The data method returns the value of the role used. If there is a relationship in the table, i search the column with relation and return the correct value.. Note that empty string is returned when the item is not found and not QVariant value, in order to have a correct visualization in QML.

void QLSqlTableModel::generateRoleNames()
{
    roles.clear();
    int nbCols = this->columnCount();

    for (int i = 0; i < nbCols; i) 
    {
        roles[Qt::UserRole + i + 1] = QVariant(this->headerData(i, Qt::Horizontal).toString()).toByteArray();
    }

#ifndef HAVE_QT5
    setRoleNames(roles);
#endif
}

The generateRoleNames method creates the roles called as the table columns specified in the header.

Example:

Tables:

query.exec("create table IF NOT EXISTS items (id integer primary key autoincrement,
            name varchar(15), descr varchar(30))");
query.exec("create table IF NOT EXISTS lista (id integer primary key autoincrement,
            qta varchar(30), item INTEGER, FOREIGN KEY (item) REFERENCES items(id))");

main.cpp

QLSqlTableModel *model = new QLSqlTableModel;
QLSqlTableModel *modelLista = new QLSqlTableModel;

model->setTable("items");
model->generateRoleNames();
model->select();

modelLista->setTable("lists");
modelLista->setRelation(2, QSqlRelation("items", "id", "name"));
modelLista->select();
modelLista->generateRoleNames();

QDeclarativeContext *ctxt = view.rootContext();
ctxt->setContextProperty("modelListItems", model);
ctxt->setContextProperty("modelList", modelLista);

In qml file

Text {
    id: name
    text: model.name
    font.bold: true; font.pointSize: 16
    color: "white"
}

Text {
    text: "Amount: " + model.qta
    font.pointSize: 16
    opacity: 1
    color: "white"
}

---

Note1: Nice post, however the performance can be tweaked. data() will be called very often. Most of the processing and object instantiation should be done only once.

Note2: I modified provided code according to this working example which was based on this wiki page