[odb-users] Persist container derived class as value type

[Henning Becker]

Am Montag, 7. März 2016, 16:03:43 schrieben Sie:
> Hi Henning,
> 
> Henning Becker <henning.becker at apworks.de> writes:
> > I'm trying to persist QPolygon from Qt as a value type. QPolygon
> > inherits from QVector<QPoint> and no data will be lost, if it is
> > handled just as a QVector<QPoint>.
> > 
> > I have defined QPoint as a composite value type:
> > #pragma db value(QPoint) transient
> > #pragma db member(QPoint::x_) virtual(int) get(x) set(setX)
> > #pragma db member(QPoint::y_) virtual(int) get(y) set(setY)
> > 
> > But how can I make odb handle QPolygon as a QVector<QPoint>?
> 
> There are two ways: "quick and dirty" and "slow but clean" ;-).
> 
> The "quick and dirty" approach is to map QPolygon to a composite value
> with a QVector<QPoint> member:
> 
> using QPolygonBase = QVector<QPoint>;
> #pragma db value(QPolygon) transient
> #pragma db member(QPolygon::points) virtual(QPolygonBase) access(this)
> 
> class Object
> {
>   ...
> 
>   QPolygon perimeter;
> };
> 
> The major drawback of this approach is that the database table name for
> perimeter will be called "object_perimeter_points" and not just
> "object_perimeter".
> 
> The "slow but clean" approach is to define an odb:container_traits<QPolygon>
> specialization, similar to the one defined from QVector<T> (in fact, you
> can just derive from it). See the Qt profile for details on how this is
> done.
Hi Boris,
thanks for your answer. It seems to work perfectly. I've written a trait, 
which just derives from QVector<QPoint>.

Here is the code, maybe anybode else needs it:
namespace odb {
  template <>
  class access::container_traits<QPolygon> : public 
access::container_traits<QVector<QPoint>>
  {};
}

Cheers,
Henning
> 
> Boris

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Henning Becker

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