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?

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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.
  • Could you explain a bit more? I've defined 2 N-1 relationships towards the "pivot table" as defined in the manual entry you quote. But I don't know where to set it up as a N:M in QT. It treats it as a 1:N Commented Mar 17, 2022 at 14:02

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