- Qt 6.12
- Qt Quick
- Important Concepts In Qt Quick - Data - Models, Views, and Data Storage
- Using SQL databases with Qt Quick Views
- Qt 6.12.0 Reference Documentation

Using C++ Models with Qt Quick Views ◦ Qt Quick Local Storage QML Types

# Using SQL databases with Qt Quick Views

## SQL Models

Qt provides C++ classes that support SQL data models. These classes work transparently on the underlying SQL data, reducing the need to run SQL queries for basic SQL operations such as create, insert, or update. For more details about these classes, see Using the SQL Model Classes.

Although the C++ classes provide complete feature sets to operate on SQL data, they do not provide data access to QML. So you must implement a C++ custom data model as a subclass of one of these classes, and expose it to QML either as a type or context property.

### Read-only Data Model

The custom model must reimplement the following methods to enable read-only access to the data from QML:

- roleNames() to expose the role names to the QML frontend. For example, the following version returns the selected table's field names as role names:

        QHash<int, QByteArray> SqlQueryModel::roleNames() const
        {
           QHash<int, QByteArray> roles;
           // record() returns an empty QSqlRecord
           for (int i = 0; i < this->record().count(); i ++) {
               roles.insert(Qt::UserRole + i + 1, record().fieldName(i).toUtf8());
           }
           return roles;
       }

- data() to expose SQL data to the QML frontend. For example, the following implementation returns data for the given model index:

       QVariant SqlQueryModel::data(const QModelIndex &index, int role) const
       {
           QVariant value;

           if (index.isValid()) {
               if (role < Qt::UserRole) {
                   value = QSqlQueryModel::data(index, role);
               } else {
                   int columnIdx = role - Qt::UserRole - 1;
                   QModelIndex modelIndex = this->index(index.row(), columnIdx);
                   value = QSqlQueryModel::data(modelIndex, Qt::DisplayRole);
               }
           }
           return value;
       }

The QSqlQueryModel class is good enough to implement a custom read-only model that represents data in an SQL database. The chat tutorial example demonstrates this very well by implementing a custom model to fetch the contact details from an SQLite database.

### Editable Data Model

QSqlTableModel provides an editable data model for a single data base table, and implements setData() as described in the Model/View Programming overview documentation.

Depending on the EditStrategy used by the model, the changes are either queued for submission later or submitted immediately.

You can also insert new data into the model by calling QSqlTableModel::insertRecord(). In the following example snippet, a QSqlRecord is populated with book details and appended to the model:

     ...
     QSqlRecord newRecord = record();
     newRecord.setValue("author", "John Grisham");
     newRecord.setValue("booktitle", "The Litigators");
     insertRecord(rowCount(), newRecord);
     ...

Using C++ Models with Qt Quick Views ◦ Qt Quick Local Storage QML Types