[Qgis-developer] Value relations in QgsExpression

[Marco Hugentobler]

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'

 >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?

Regards,
Marco

On 28.07.2014 12:02, Matthias Kuhn wrote:
> Hi Marco,
>
> On 28.07.2014 11:57, Marco Hugentobler wrote:
>> Hi
>>
>> In QgsVectorLayer, it is possible to define the edittype 'value
>> relation' where a key is replaced by a value in a related layer. Now
>> I'd like to make a function in QgsExpression that behaves similarly
>> and returns the value in the related table.
>>
>> My idea is to create a function 'valueRelation( value, layer_id,
>> key_field, value_field )' to return that value, where layer_id is the
>> id of the related layer, key_file and value_field the column names in
>> the related layer.
>>
>> One disadvantage of this approach is that if the relation in
>> vectorlayer changes, the one in an expression needs to be changed as
>> well. On the other hand it can be an advantage in certain situations
>> to have a different relation in expression than in vector layer. I
>> figured out it would be difficult to take the value relation
>> dynamically from vectorlayer, as QgsExpression does not know its
>> vectorlayer. A possibility would be to create a special column for the
>> current layer id. However that seems not nice and it is too easy to
>> forget to set that id when using the expression.
>>
>> What are your opinions on that? Are there better ways to accomplish
>> the value relation function in QgsExpression?
>>
>> Regards,
>> Marco
>>
> Does this introduce any additional functionality compared to defining a
> layer join and using the fields appended to the vector layer be this?
>
> 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.
>
> 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.
>
> Regards,
> Matthias


-- 
Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee