Populating a column of type PostgreSQL array and list widget with an expression fails if "Apply default value on update" is enabled

[andreasneumann]

What is the bug or the crash?

The setup contains a PostgreSQL table where one or more columns are of type array (array of int4 or array of string).

CREATE TABLE test.test (
	t_id serial,
	geometrie geometry(multipolygon, 2056) NOT NULL,
	bfs_nr int4[],
	gb_nr varchar[],
	flurname varchar(255)
);

The associated widget should be a "list widget" and the list should be populated with an "overlay_intersects()" expression function as a default value:

The problem is that I cannot populate the list widget with the QGIS expression. The list always stays empty, even if I can see a correct preview that shows that the expression works.

I tried the following three expressions:

overlay_intersects(layer:='hoheitsgrenzen_gemeindegrenze', expression:=bfs_gemeindenummer)

'[' ||
array_to_string(
overlay_intersects(layer:='hoheitsgrenzen_gemeindegrenze', expression:=bfs_gemeindenummer),
',')
|| ']'

'{' ||
array_to_string(
overlay_intersects(layer:='hoheitsgrenzen_gemeindegrenze', expression:=bfs_gemeindenummer),
',')
|| '}'

Neither of the three expressions was able to populate the list widget, even if the preview always contained correct data.

Note: PostgreSQL arrays use curly brackets ( {} ) while QGIS arrays use square brackets ( [] ).

Steps to reproduce the issue

Dataset to test against (Geopackage) - for the overlay_intersects() expression:

hoheitsgrenzen_gemeindegrenze.gpkg.zip
.

Versions

<style type="text/css"> p, li { white-space: pre-wrap; } </style>

QGIS version	3.23.0-Master	QGIS code revision	1d1d59d
Qt version	5.12.8
Python version	3.8.10
GDAL/OGR version	3.3.1
PROJ version	8.1.0
EPSG Registry database version	v10.027 (2021-06-17)
GEOS version	3.9.1-CAPI-1.14.2
SQLite version	3.31.1
Compiled against PDAL	2.0.1	Running against PDAL	2.3.0
PostgreSQL client version	13.3
SpatiaLite version	5.0.1
QWT version	6.1.4
QScintilla2 version	2.11.2
OS version	Ubuntu 20.04.3 LTS
Active Python plugins
changeDataSource	3.1
trackable_project_files	v1.2.2
QgisModelBaker	v6.6.7
QuickWKT	3.1
DataPlotly	3.8.1
sagaprovider	2.12.99
processing	2.12.99
db_manager	0.1.20
grassprovider	2.12.99
solocator	1.4.1

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[m-kuhn]

Interestingly, this works for me with QGIS master, the postgis layer (created with the SQL in the issue description) and a simple scratch layer that corresponds to hoheitsgrenzen_gemeindegrenze.
This works if the data type of the field is int or string (with string the provider doesn't let me save the edits but the expression works and the widget is filled nonetheless).

Attached you find a gpkg project and a .qgs project (if you want to edit the XML before loading)
Can you test if this also works with your setup?

[andreasneumann]

Confirmed - with your project it works, with my project not.

Now I have to dig in to find what the differences are ...

Let's keep the ticket open until it also works in my projects. I will add more info.

[andreasneumann]

Ok - this is really weird.

If you rename the attribute "x" in layer "array_test" it starts failing. It would still display in the "Identify Results" panel correctly, but not in the list widgets.

Perhaps QGIS doesn't like it if both layers have the same attribute name?

[andreasneumann]

Oh no - it is some completely different issue. Not the naming issue.

It starts failing if you enable the "Apply default value on update"

It has nothing to do with the column names.

[m-kuhn]

Fix pending in #46172

FYI, the default value expression needs to return an array (not a string representation of an array), the fix is in the update of the default value on the attribute dialog, specific for "apply on update" and will work for any datatype that is not trivially convertible to string and back (like arrays here, but could also be geometry, date, ...).