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I cannot figure out why, but, in a tree/planting projects database, I have a fairly simply data model where I have one polygon layer to symbolize the area/project (eg. Maple Oak Park, John Smith School), and a point layer to symbolize the trees planted in that area/project. I created a relation between the two layers using the area/project name as the join field (image #1).

I want to then add this relation to the form for the area/project layer, so that all the trees associated to the area/project appear on the form. See image #3 for an example where this worked for me a few months ago on another set of layers. However, for some reason the relation simply does not appear as an option in the attributes form (image #2).

https://i.stack.imgur.com/GTGeE.jpg

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You did create a join, not a relation.

Joins and a relations are different things. A join (layer based) does add attributes from another layer based on a 1:1 or 1:0 cardinality relationship, but in the described case you have a 1:n relationship (1 project : n trees).

The behavior you need can be achieved via Project > Properties... > Relations. Add a new relation, give it a name and define your key fields (project = parent, tree/plant = child):

enter image description here

... and this relation will soon appear in the form dialog.

This works for whatever data source, no matter if shapefiles or PostGIS.

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