qt5base-lts/examples/sql/doc/src/sqlwidgetmapper.qdoc

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/*!
    \example sqlwidgetmapper
    \title SQL Widget Mapper Example
    \ingroup sql_examples

    \brief The SQL Widget Mapper example shows how to use a map information from a
    database to widgets on a form.

    \borderedimage sql-widget-mapper.png

    In the \l{Combo Widget Mapper Example}, we showed how to use a named
    mapping between a widget mapper and a QComboBox widget with a special
    purpose model to relate values in the model to a list of choices.

    Again, we create a \c Window class with an almost identical user interface,
    providing a combo box to allow their addresses to be classified as "Home",
    "Work" or "Other". However, instead of using a separate model to hold these
    address types, we use one database table to hold the example data and
    another to hold the address types. In this way, we store all the
    information in the same place.

    \section1 Window Class Definition

    The class provides a constructor, a slot to keep the buttons up to date,
    and a private function to set up the model:

    \snippet sqlwidgetmapper/window.h Window definition

    In addition to the QDataWidgetMapper object and the controls used to make
    up the user interface, we use a QStandardItemModel to hold our data and
    a QStringListModel to hold information about the types of address that
    can be applied to each person's data.

    \section1 Window Class Implementation

    The first act performed by the \c Window class constructor is to set up
    the model used to hold the example data. Since this is a key part of the
    example, we will look at this first.

    The model is initialized in the window's \c{setupModel()} function. Here,
    we create a SQLite database containing a "person" table with primary key,
    name, address and type fields.

    \snippet sqlwidgetmapper/window.cpp Set up the main table

    On each row of the table, we insert default values for these fields,
    including values for the address types that correspond to the address
    types are stored in a separate table.

    \borderedimage widgetmapper-sql-mapping-table.png

    We create an "addresstype" table containing the identifiers used in the
    "person" table and the corresponding strings:

    \snippet sqlwidgetmapper/window.cpp Set up the address type table

    The "typeid" field in the "person" table is related to the contents of
    the "addresstype" table via a relation in a QSqlRelationalTableModel.
    This kind of model performs all the necessary work to store the data in
    a database and also allows any relations to be used as models in their
    own right.

    In this case, we have defined a relation for the "typeid" field in the
    "person" table that relates it to the "id" field in the "addresstype"
    table and which causes the contents of the "description" field to be
    used wherever the "typeid" is presented to the user. (See the
    QSqlRelationalTableModel::setRelation() documentation for details.)

    \borderedimage widgetmapper-sql-mapping.png

    The constructor of the \c Window class can be explained in three parts.
    In the first part, we set up the model used to hold the data, then we set
    up the widgets used for the user interface:

    \snippet sqlwidgetmapper/window.cpp Set up widgets

    We obtain a model for the combo box from the main model, based on the
    relation we set up for the "typeid" field. The call to the combo box's
    \l{QComboBox::}{setModelColumn()} selects the field in the field in the
    model to display.

    Note that this approach is similar to the one used in the
    \l{Combo Widget Mapper Example} in that we set up a model for the
    combo box. However, in this case, we obtain a model based on a relation
    in the QSqlRelationalTableModel rather than create a separate one.

    Next, we set up the widget mapper, relating each input widget to a field
    in the model:

    \snippet sqlwidgetmapper/window.cpp Set up the mapper

    For the combo box, we already know the index of the field in the model
    from the \c{setupModel()} function. We use a QSqlRelationalDelegate as
    a proxy between the mapper and the input widgets to match up the "typeid"
    values in the model with those in the combo box's model and populate the
    combo box with descriptions rather than integer values.

    As a result, the user is able to select an item from the combo box,
    and the associated value is written back to the model.

    The rest of the constructor is very similar to that of the
    \l{Simple Widget Mapper Example}:

    \snippet sqlwidgetmapper/window.cpp Set up connections and layouts

    We show the implementation of the \c{updateButtons()} slot for
    completeness:

    \snippet sqlwidgetmapper/window.cpp Slot for updating the buttons

    \omit
    \section1 Delegate Class Definition and Implementation

    The delegate we use to mediate interaction between the widget mapper and
    the input widgets is a small QItemDelegate subclass:

    \snippet sqlwidgetmapper/delegate.h Delegate class definition

    This provides implementations of the two standard functions used to pass
    data between editor widgets and the model (see the \l{Delegate Classes}
    documentation for a more general description of these functions).

    Since we only provide an empty implementation of the constructor, we
    concentrate on the other two functions.

    The \l{QItemDelegate::}{setEditorData()} implementation takes the data
    referred to by the model index supplied and processes it according to
    the presence of a \c currentIndex property in the editor widget:

    \snippet sqlwidgetmapper/delegate.cpp setEditorData implementation

    If, like QComboBox, the editor widget has this property, it is set using
    the value from the model. Since we are passing around QVariant values,
    the strings stored in the model are automatically converted to the integer
    values needed for the \c currentIndex property.

    As a result, instead of showing "0", "1" or "2" in the combo box, one of
    its predefined set of items is shown. We call QItemDelegate::setEditorData()
    for widgets without the \c currentIndex property.

    The \l{QItemDelegate::}{setModelData()} implementation performs the reverse
    process, taking the value stored in the widget's \c currentIndex property
    and storing it back in the model:

    \snippet sqlwidgetmapper/delegate.cpp setModelData implementation
    \endomit

    \section1 Summary and Further Reading

    The use of a separate model for the combo box and a special delegate for the
    widget mapper allows us to present a menu of choices to the user. Although
    the choices are stored in the same database as the user's data, they are held
    in a separate table. Using this approach, we can reconstructed complete records
    at a later time while using database features appropriately.

    If SQL models are not being used, it is still possible to use more than
    one model to present choices to the user. This is covered by the
    \l{Combo Widget Mapper Example}.
*/