Return generated geometry using a QGIS aggregate function

Question

I have a "Phenomenon" table, listing the location (Province, or admin level 3 in this case) of where a certain phenomenon was observed. I would like to return the centroid geometry from a Provinces polygon layer to this Phenomenon table using a QGIS aggregate function.

I have standardized columns with shared PCodes (admin3Pcode OR adm3Pcode) between Provinces and Phenomenon tables.

I am not getting any output from the following aggregate function and I don't understand why.

aggregate(
 layer:= 'adm3_polys', 
 aggregate:='array_agg',
 expression:=geom_to_wkt(centroid(@geometry)),
 filter:='adm3Pcode'= attribute(@parent, 'admin3Pcode')
 )

Answer 1

You can put the geometry in a field, but it will be more complex. I suggest you to create two fields: x_centroid and y_centroid.

Try this:

To calculate x:

aggregate(
    layer:= 'adm3_polys', 
    aggregate:='array_agg',
    expression:=x(centroid(@geometry)),
    filter:="adm3Pcode"= attribute(@parent, 'admin3Pcode')
)[0]

To calculate y:

aggregate(
    layer:= 'adm3_polys', 
    aggregate:='array_agg',
    expression:=y(centroid(@geometry)),
    filter:="adm3Pcode"= attribute(@parent, 'admin3Pcode')
)[0]

And be careful with the double quotation, it's a little bit confusing. For example: here filter:="adm3Pcode"= attribute(@parent, 'admin3Pcode')) you use double quotes for "adm3Pcode" because we are referring a field, but here attribute(@parent, 'admin3Pcode') you use single quotes because the attribute() function takes the name of the field as a string.