source: trunk/doc/src/porting/qt4-sql.qdoc@ 846

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/*!
    \page qt4-sql.html
    \title The Qt 4 Database GUI Layer

    \contentspage {What's New in Qt 4}{Home}
    \previouspage Cross-Platform Accessibility Support in Qt 4
    \nextpage The Network Module in Qt 4

    The GUI layer of the SQL module in Qt 4 has been entirely
    redesigned to work with \l{qt4-interview.html}{Interview} (Qt's
    new model/view classes). It consists of three model classes
    (QSqlQueryModel, QSqlTableModel, and QSqlRelationalTableModel)
    that can be used with Qt's view classes, notably QTableView.

    \section1 General Overview

    The Qt 4 SQL classes are divided into three layers:

    \list
    \o The database drivers
    \o The core SQL classes
    \o The GUI classes
    \endlist

    The database drivers and the core SQL classes are mostly the same
    as in Qt 3. The database item models are new with Qt 4; they
    inherit from QAbstractItemModel and make it easy to present data
    from a database in a view class such as QListView, QTableView,
    and QTreeView.

    The philosophy behind the Qt 4 SQL module is that it should be
    possible to use database models for rendering and editing data
    just like any other item models. By changing the model at
    run-time, you can decide whether you want to store your data in
    an SQL database or in, say, an XML file. This generic approach
    has the additional benefit that you don't need to know anything
    about SQL to display and edit data.

    The Qt 4 SQL module includes three item models:

    \list
    \o QSqlQueryModel is a read-only model based on an arbitrary
       SQL query.
    \o QSqlTableModel is a read-write model that works on a single
       table.
    \o QSqlRelationalTableModel is a QSqlTableModel subclass with
       foreign key support.
    \endlist

    Combined with Qt's view classes and Qt's default delegate class
    (QItemDelegate), the models offer a very powerful mechanism for
    accessing databases. For finer control on the rendering of the
    fields, you can subclass one of the predefined models, or even
    QAbstractItemDelegate or QItemDelegate if you need finer control.

    You can also perform some customizations without subclassing. For
    example, you can sort a table using QSqlTableModel::sort(), and
    you can initialize new rows by connecting to the
    QSqlTableModel::primeInsert() signal.

    One nice feature supported by the read-write models is the
    possibility to perform changes to the item model without
    affecting the database until QSqlTableModel::submitAll() is
    called. Changes can be dropped using QSqlTableModel::revertAll().

    The new classes perform advantageously compared to the SQL
    module's GUI layer in Qt 3. Speed and memory improvements in the
    tool classes (especially QVariant, QString, and QMap) and in the
    SQL drivers contribute to making Qt 4 database applications more
    snappy.

    See the \l QtSql module overview for a more complete introduction
    to Qt's SQL classes.

    \section1 Example Code

    The simplest way to present data from a database is to simply
    combine a QSqlQueryModel with a QTableView:

    \snippet doc/src/snippets/code/doc_src_qt4-sql.qdoc 0

    To present the contents of a single table, we can use
    QSqlTableModel instead:

    \snippet doc/src/snippets/code/doc_src_qt4-sql.qdoc 1

    In practice, it's common that we need to customize the rendering
    of a field in the database. In that case, we can create our own
    model based on QSqlQueryModel. The next code snippet shows a
    custom model that prepends '#' to the value in field 0 and
    converts the value in field 2 to uppercase:

    \snippet examples/sql/querymodel/customsqlmodel.h 0
    \codeline
    \snippet examples/sql/querymodel/customsqlmodel.cpp 0

    It is also possible to subclass QSqlQueryModel to add support for
    editing. This is done by reimplementing
    QAbstractItemModel::flags() to specify which database fields are
    editable and QAbstractItemModel::setData() to modify the
    database. Here's an example of a setData() reimplementation that
    changes the first or last name of a person:

    \snippet examples/sql/querymodel/editablesqlmodel.cpp 1

    It relies on helper functions called \c setFirstName() and
    \c setLastName(), which execute an \c{update}. Here's
    \c setFirstName():

    \snippet examples/sql/querymodel/editablesqlmodel.cpp 2

    See Qt's \c examples/sql directory for more examples.

    \section1 Comparison with Qt 3

    The core SQL database classes haven't changed so much since Qt 3.
    Here's a list of the main changes:

    \list
    \o QSqlDatabase is now value-based instead of pointer-based.
    \o QSqlFieldInfo and QSqlRecordInfo has been merged into
       QSqlField and QSqlRecord.
    \o The SQL query generation has been moved into the drivers. This
       makes it possible to use non-standard SQL extensions. It also
       opens the door to non-SQL databases.
    \endlist

    The GUI-related database classes have been entirely redesigned.
    The QSqlCursor abstraction has been replaced with QSqlQueryModel
    and QSqlTableModel; QSqlEditorFactory is replaced by
    QAbstractItemDelegate; QDataTable is replaced by QTableView. The
    old classes are part of the \l{Qt3Support} library to aid
    porting to Qt 4.
*/