QSqlQueryModel eager loading example

main.cpp

//based on https://doc.qt.io/qt-5/qsqlquerymodel.html#fetchMore
#include <QtWidgets>
#include <QtSql>
#include <type_traits>

//comment out the next line to disable eager loading
#define QSQLMODEL_EAGER_LODING 1

//a class template that can be used to disable lazy loading
//in any QAbstractTableModel subclass
template<typename TableModel>
struct EagerlyLoaded : public TableModel {
    static_assert(std::is_base_of<QAbstractTableModel, TableModel>::value,
                  "EagerlyLoaded works only with QAbstractTableModel subclasses");
    explicit EagerlyLoaded(QObject* parent = nullptr):TableModel(parent) {
        QObject::connect(this, &QAbstractTableModel::modelReset,
                         [this]{ while(this->canFetchMore()) this->fetchMore(); });
    }
};

int main(int argc, char* argv[]) {
    QApplication a(argc, argv);

    //setup an in-memory database table and fill it with dummy data
    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName(":memory:");
    if(!db.open()) return 1;
    QSqlQuery query;
    query.prepare("CREATE TABLE test_table(id PRIMARY KEY, name TEXT);");
    if(!query.exec()) return 2;
    query.prepare("INSERT INTO test_table(name) VALUES(?);");
    QVariantList values;
    values << "aa"; // <<-- suppose that this is an interesting value to search for
    for(int i=0; i<100000; i++) values << "bb"; //100k rows of dull values
    values << "aa"; // another interestng value at the end
    query.addBindValue(values);
    if(!query.execBatch(QSqlQuery::ValuesAsRows)) return 3;

    //setup GUI
    QWidget w;
    QVBoxLayout layout(&w);
    QLineEdit lineEdit;
    QTableView tableView;
    layout.addWidget(&lineEdit);
    layout.addWidget(&tableView);

    //setup models
#ifdef QSQLMODEL_EAGER_LODING
    EagerlyLoaded<QSqlQueryModel> model;
#else
    QSqlQueryModel model;
#endif
    QSortFilterProxyModel filterModel;
    model.setQuery("SELECT * FROM test_table ORDER BY id;");
    filterModel.setSourceModel(&model);
    filterModel.setFilterKeyColumn(1);
    tableView.setModel(&filterModel);
    //hookup the line edit with the filter model
    QObject::connect(&lineEdit, &QLineEdit::textChanged,
                     &filterModel, &QSortFilterProxyModel::setFilterFixedString);
    //set the line edit to the interesting value
    lineEdit.setText("aa"); //this should trigger the filter model

    w.show();
    return a.exec();
}