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/*!

    \example sql/sqlwidgetmapper

    \title SQL Widget Mapper Example

    The SQL Widget Mapper example shows how to use a map information from a

    database to widgets on a form.

    \image 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 examples/sql/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 examples/sql/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.

    \image widgetmapper-sql-mapping-table.png

    We create an "addresstype" table containing the identifiers used in the

    "person" table and the corresponding strings:

    \snippet examples/sql/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.)

    \image 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 examples/sql/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 examples/sql/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 examples/sql/sqlwidgetmapper/window.cpp Set up connections and layouts

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

    completeness:

    \snippet examples/sql/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 examples/sql/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 examples/sql/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 examples/sql/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}.

*/