N:M relation in QGIS with custom form in Qt Designer

Question

The manual: https://docs.qgis.org/3.16/en/docs/user_manual/working_with_vector/attribute_table.html#creating-one-or-many-to-many-relations describes how to create a N:M relation and how to use it with a attribute form made with drag and drop designer.

With Qt Designer I am only able to create a form for a 1:N relation using a QgsRelationEditorWidget and set the qgisRelation property accordingly. But how do I use a QgsRelationEditorWidget for a N:M (many to many) relation?

Answer 1

I found out that creating a form for a N:M (many to many) relation in Qt Designer works in the same way as for a 1:N (one to man) relation:

1. Add a QgsRelationEditorWidget.
2. Set the qgisRelation property to the Id of your relation (as seen in QGIS -> Project Properties -> Relations). My fault was that I was using the relation name rather than the Id, which are identical for 1:N relations (if automatically created).
3. Don't forget to check the option Allow adding new features in the QGIS attribute forms dialogue in your associative table.