[QGIS-Developer] N:M relations seem broken

[Matteo Ghetta]

Hi all,

after some tests made on a custom geopackage, but the same happens also 
with Postgres, it seems to me that N:M realtions, when adding a child, 
are broken.

After playing a little bit, adding existing children to the parent works 
fine, while the problem is when adding a new child from the parent form: 
the id of the child foreign key seems to be saved immediately when 
opening the form and the final result is an empty value in the pivot table.

Also the exercise in the documentation seems not working for both 
postgres (same error described above) and geopackage. 
(https://docs.qgis.org/3.34/en/docs/user_manual/working_with_vector/joins_relations.html#many-to-many-n-m-relations)

I opened a ticket with a small example:

https://github.com/qgis/QGIS/issues/59546

Thanks for any feedback

Cheers

Matteo