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