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I have a PostgreSQL/PostGIS database with a base table (Pois_points) and two subtables which constitute a many-to-many relationship with the base table, as you can see on the picture below (click on Many to many relationships).

I want to build a QGIS form that depicts this relationship between the tables when adding and editing items. I have seen examples of a one-to-many relationship in QGIS, which is relatively easy. But many-to-many is not documented or maybe not possible at all?

Many to many relationships

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    Could you please add QGIS version & OS? – nielsgerrits Aug 12 '16 at 8:22
  • I'm sorry: QGis version 2.16.1, installed on Windows 10, 64bits – Pim Verver Aug 13 '16 at 11:25
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If you don't mind having a bit of bad design in your database, you can create the relation between your Pois_points and History tables using the relational value widget and check "allow multiple selections". You'll have to create a new column in your Pois_points table in which foreign keys will be stored as varchar arrays.

If you want a clean N:M relationship you can try this solution but you'll have to write some Python.

Since this pull request it should now be possible to create N:M relationship in QGIS easily. Documentation can be found here.

Unfortunately I can't make it working so I opened a ticket.

N:M relationship configuration

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Try creating a relationship for poi-pos and the ps-points-history table, then a relationship for the history table and pos-points-history table.

  • The relationships themselves are allready defined on the database level. What I am looking for is how to make a QGIS form of the attributes of each feature displayed in QGIS, where for instance the selected history names are viewable and where you can change the selection based of a dropdown list of available history names. If its a one-to-many relationship, that's not so difficult, if it's a many-to-many then I haven't seen any documented working example. I have tried on my own, but without any satisfying result. – Pim Verver Aug 13 '16 at 11:30
  • Yeh what I'm saying is in QGIS you define both relationships separately, then set up the one form and you should be right. From your question I assume you can already setup a relationship in QGIS and set up the forms, if not I can give you help on tha – Liam G Aug 13 '16 at 11:40

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