Fetch multiple fields in relation_aggregate expression

Question

In the QGIS 3.34+ expression builder, how could one use the expression parameter in the relation_aggregate to fetch multiple fields?

E.g.:

relation_aggregate( 
    relation:= 'my_relation', 
    aggregate:= 'array_agg', 
    expression:= "rep_lyr,rep_id",
    concatenator:= "; "
)

but this doesn't work.

The goal is to fetch related data in a many to many join table as follow:

id	ref_id	rep_lyr	rep_id
1	1	lyrB	2
2	1	lyrC	3
3	4	lyrB	7
4	5	lyrA	8
5	9	lyrB	11
6	7	lyrB	11

I'm trying to set up labels for my features:

Answer 1

One option is to concatenate the field values like so:

relation_aggregate( 
    relation:= 'my_relation', 
    aggregate:= 'array_agg', 
    -- field values concatenated into a string separated by a comma
    expression:= "rep_lyr" || ',' || "rep_id"
)