Add child feature in N:M relation not working correctly: child id not saved in pivot table

[ghtmtt]

What is the bug or the crash?

When adding a child feature the child id is not saved into the pivot table while the parent id is correctly saved. I'm attaching a small project to reproduce the issue: relation_bug.zip

It seems that the edits on the relation layer are saved when opening the form of the child feature and not when clicking on OK (you can notice it thanks to the editing icon in the legend). Therefore, when finally saving all the edits, the ones made on the relation are not stored.

Ref also to https://github.com/qgis/QGIS/pull/57689?

Screencast 2024-11-20 15:20:38.webm

Steps to reproduce the issue

The N:M relation is between the points and projects layer with the relation layer that is the pivot table.

• identify a feature on the points layer

• in the embedded form toggle the editing mode for the child (project layer)

  

• type a new ID (in the attached projects existing ids of the projects table are 1, 2, 3) in the upper part of the new form

  

• click OK and save

While the new id of is correctly saved into the projects layer, it is not saved into the relation layer (the N:M):

Versions

QGIS LTR 3.34 but also QGIS master

QGIS version	3.41.0-Master
QGIS code revision	f7b21861e26
Libraries
Qt version	5.15.8
Python version	3.11.2
GDAL/OGR version	3.6.2
PROJ version	9.1.1
EPSG Registry database version	v10.076 (2022-08-31)
GEOS version	3.11.1-CAPI-1.17.1
SQLite version	3.40.1
PostgreSQL client version	15.8 (Debian 15.8-0+deb12u1)
SpatiaLite version	5.0.1
QWT version	6.1.4
QScintilla2 version	2.13.3
OS version	Debian GNU/Linux 12 (bookworm)
Active Python plugins
lizmap	master
plugin_comune_senigallia	1.0.0
pluginbuilder3	3.2.1
QuickOSM	2.2.3
faunalia_toolkit	0.1.0
line_plan_survey	0.1.0
osminfo	1.1.0
plugin_reloader	0.17
DataPlotly	master
SpreadsheetLayers	2.1.2
processing	2.12.99
grassprovider	2.12.99
MetaSearch	0.3.6
db_manager	0.1.20

Supported QGIS version

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

New profile

• [X] I tried with a new QGIS profile

Additional context

No response