1:n relations in label expressions & general syntax

Question

I've got a colleague who wants to create a (concatenated) label based on a 1:n relation between some shapes and related tables. They asked me if this was possible in QGIS - and I think it should.

I found this: QGIS: relations in label expressions

However, I failed to generalise this for 1:n and m:n relations. The solution provided by @ben-w isn't using the relation, and the solution by @alexander-novikov uses relation_aggregate() which is not really suitable for simply just getting multiple character values our of a relation.

Can someone point me to to right direction, and maybe provide some syntax explanation?

(NB: Sorry not to provide a reproducible example. I not using QGIS expressions normally, I am coming from R using {dplyr}, and I don't speak SQL, nor Python. Different syntax, maybe sometimes different logic. R is no solution for my colleague, so I try to learn something new.)

Answer 1

You can use the aggregate function in the expression editor:

 aggregate( layer:= 'N_LAYER',
            aggregate:='concatenate', 
            expression:="FIELDNAME_IN_N_LAYER",    
            concatenator:=', ',
            filter:=attribute(  $currentfeature ,'FOREIGNKEY_IN_N_LAYER')=attribute(  @parent ,'KEY_IN_1_LAYER'))

where you tell the aggregate function in the filter criteria the relation between the key/foreign key. Be aware that $currentfeature belongs to the n_layer and @parent to the 1_layer in your 1:n relation.