[Question]: correct attribute form setup for area calculations? (area($geometry)

[timbo-barnes]

What is the question?

I've tried every combination of attribute form layout to try and write the area value of an added or edited polygon to the attribute table without success.
My layer is a multipolygon with a column called area_ha of double (int8) type. The attribute form for this field uses a Text Edit widget.
I've set up layer edition (editing) on this layer in the plugin- All the other editing tasks work fine (i.e. adding/editing geometry, changing other attributes)

• The issue isn't adding attributes themselves (other fields works, including date input to a date field using/setting the default to now())
• The area calculations work fine in QGIS
• I've tried different methods (i.e. format_number(area($geometry)/10000,2) vs round($area/10000,2))
• I've tried an 'editable' field in the form and not
• I've tried 'Apply default value on update' or not.

Nothing works- Has anyone had this working?

Data is in Postgis as a multipolygon, 2193 (New Zealand Transverse Mercator)

Versions

Versions :

• Lizmap Web Client : 3.6.3
• Lizmap plugin : 3.16.4
• QGIS Desktop : 3.30.1
• QGIS Server : 3.30.3
• Py-QGIS-Server : not used
• QGIS Server plugin atlasprint : 3.3.2
• QGIS Server plugin lizmap_server : 2.7.1
• QGIS Server plugin wfsOutputExtension : 1.7.1

Check Lizmap plugin

• I have done the step just before in the Lizmap QGIS desktop plugin before opening this ticket. Otherwise, my ticket is not considered valid and might get closed.

QGIS server version, only if the section above doesn't mention the QGIS Server version

No response

Operating system

Ubuntu Linux 22.04.2

Browsers

Chrome

Browsers version

Chrome Version 117.0.5938.89 (Official Build) (64-bit)

Relevant log output

No response

[gioman]

Nothing works- Has anyone had this working?

@timbo-barnes my understanding is that doesn't work because the attribute dialog form is shown before the geometry is actually drawn and finished (while in QGIS the user always first draws a feature and then the attributes dialog shows up).

[Gustry]

For sure "Apply on update" is not supported.

Please have a look to the discussion #3226

Some documentation is needed to write what is supported in LWC about expressions in forms.

[Gustry]

Both in your web browser and QGIS server, check HTTP requests, you should see the "SERVICE=EXPRESSIONS" requests to evaluate expressions.

[timbo-barnes]

OK, I've got around this by using a database trigger on insert or update which works- Perhaps the documentation could be updated to note that use of geom field and/or Apply on Update isn't supported currently?

[Gustry]

Perhaps the documentation could be updated to note that use of geom field and/or Apply on Update isn't supported currently?

Yes of course, the documentation is on GitHub as well https://github.com/3liz/lizmap-documentation 😉
Feel free to provide a PR