[Qgis-user] Create virtual fields with pyQGIS with standalone scripts

[Roland Berger]

Hi Hannes and Ujaval

I found the problem. After adding a virtual field I also loaded style 
from a qml file for that layer:

layer.loadNamedStyle("mystyle.qml", QgsMapLayer./Symbology/)

Looks like even when I only load the stye for the /Symbology/ also the 
/Fields/ settings are somehow changed/overwritten.

Thanks
Roland



On 7/26/24 16:39, Johannes Kröger (WhereGroup) via QGIS-User wrote:
>
> QGIS' virtual fields (expression fields) are not 
> https://www.postgresql.org/docs/current/ddl-generated-columns.html
>
> They are conceptually similar between QGIS and PostgreSQL, but they 
> are not propagated from QGIS to the data source as a new column there. 
> They live only in the QGIS project.
>
> If you want a generated column in your PostgreSQL database, you need 
> to add it another way. E.g. via an SQL query.
>
> Cheers, Hannes
>
> On 26.07.24 12:46, Roland Berger via QGIS-User wrote:
>> Hi
>>
>> I would like to add a virtual field to a PostGIS vector layer with 
>> PyQGIS with standalone scripts 
>> (https://docs.qgis.org/3.34/en/docs/pyqgis_developer_cookbook/intro.html#python-applications)
>> All examples I have found so far with google, chatgpt4 and JetBrains 
>> AI Assistant gave me solutions that do not work. They only work in 
>> the Python Console inside QGIS.
>> Does anybody know of a solution that works with PyQGIS with 
>> standalone scripts?
>> See below what I have tried so far in different variations.
>>
>> Thanks for any help
>> Roland
>>
>> Example created by chatgpt:
>> ...
>> field_name = "virtual_field"
>> expression = '"attribute_name" * 2'  # Example expression
>> # Create and add the virtual field
>> layer.startEditing()
>> layer.addExpressionField(expression, QgsField(field_name, QVariant.Double))
>> layer.commitChanges()
>> ...
>>
>>
>>
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