[Qgis-developer] Value relations in QgsExpression

[Matthias Kuhn]

On 28.07.2014 13:49, Marco Hugentobler wrote:
> Hi Matthias
>
>> Do you think it would be possible to introduce this via a JOIN syntax in
>> QgsExpression? This way we would stay closer to SQL standard.
>
> That's a good idea, I also think JOIN syntax is better.
> The return of a JOIN operation is probably a tuple, so by using that
> as a QVariant, it might be possible to use something like 'JOIN(
> layer_id) ON (fieldA = fieldB).$attribute'

What do you think about this notation?

layer_id.attr1 + layer_id.attr2 JOIN layer_id on fieldA = fieldB

The tuple would in QGIS terms be a QgsFeature I guess.

Now thinking in a more generic way, a JOIN would JOIN two tables and not
features. In that case (we possibly might end up supporting this kind of
virtual tables e.g. from python plugins) a JOIN would then return a
QgsFeatureIterator wrapped in a QVariant instead of a single tuple.
I am not sure how exactly such a feature would be integrated yet, and
how this interacts with the current expression engine (there are a
couple of shared functionalities, but also some that are different),
therefore I am also not sure if this should have any implications on the
current implementation. So this paragraph is just thinking out lout.

>
> >Another possibility I was thinking of was via the relations, with the
> >possibility to check some fields of a related layer as "hard joined"
> >which would then be available the same way that JOINED fields are now.
>
> Would that basically be a join limited to certain attributes or would
> the handling be totally different to the layer join mechanism?
That would be a join limited to certain attributes and my idea is that
this would eventually be merged with the current JOIN functionality to
remove duplicated work in this area.

Regards
Matthias