Cached Table Example▲

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The example consists of a single class, TableEditor, which is a custom dialog widget that allows the user to modify data stored in a database. We will first review the class definiton and how to use the class, then we will take a look at the implementation.

TableEditor Class Definition▲

The TableEditor class inherits QDialog making the table editor widget a top-level dialog window.

 
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class TableEditor : public QWidget
{
    Q_OBJECT

public:
    explicit TableEditor(const QString &tableName, QWidget *parent = nullptr);

private slots:
    void submit();

private:
    QPushButton *submitButton;
    QPushButton *revertButton;
    QPushButton *quitButton;
    QDialogButtonBox *buttonBox;
    QSqlTableModel *model;
};

The TableEditor constructor takes two arguments: The first is a pointer to the parent widget and is passed on to the base class constructor. The other is a reference to the database table the TableEditor object will operate on.

Note the QSqlTableModel variable declaration: As we will see in this example, the QSqlTableModel class can be used to provide data to view classes such as QTableView. The QSqlTableModel class provides an editable data model making it possible to read and write database records from a single table. It is build on top of the lower-level QSqlQuery class which provides means of executing and manipulating SQL statements.

We are also going to show how a table view can be used to cache any changes to the data until the user explicitly requests to submit them. For that reason we need to declare a submit() slot in additon to the model and the editor's buttons.

Connecting to a Database

Before we can use the TableEditor class, we must create a connection to the database containing the table we want to edit:

 
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int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    if (!createConnection())
        return 1;

    TableEditor editor("person");
    editor.show();
    return app.exec();
}

The createConnection() function is a helper function provided for convenience. It is defined in the connection.h file which is located in the sql example directory (all the examples in the sql directory use this function to connect to a database).

 
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static bool createConnection()
{
    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName(":memory:");
    if (!db.open()) {
        QMessageBox::critical(nullptr, QObject::tr("Cannot open database"),
            QObject::tr("Unable to establish a database connection.\n"
                        "This example needs SQLite support. Please read "
                        "the Qt SQL driver documentation for information how "
                        "to build it.\n\n"
                        "Click Cancel to exit."), QMessageBox::Cancel);
        return false;
    }

    QSqlQuery query;
    query.exec("create table person (id int primary key, "
               "firstname varchar(20), lastname varchar(20))");
    query.exec("insert into person values(101, 'Danny', 'Young')");
    query.exec("insert into person values(102, 'Christine', 'Holand')");
    query.exec("insert into person values(103, 'Lars', 'Gordon')");
    query.exec("insert into person values(104, 'Roberto', 'Robitaille')");
    query.exec("insert into person values(105, 'Maria', 'Papadopoulos')");

    query.exec("create table items (id int primary key,"
                                             "imagefile int,"
                                             "itemtype varchar(20),"
                                             "description varchar(100))");
    query.exec("insert into items "
               "values(0, 0, 'Qt',"
               "'Qt is a full development framework with tools designed to "
               "streamline the creation of stunning applications and  "
               "