labeling - QGIS: relations in label expressions

In my project I have a many-to-one relationship.

For example I have a vector layer "Towns" and a data layer "TownsInfo".

Each town record consists of "Name" and "Id". Each TownsInfo record consist of "Id", "TownId", "Population" and "Source".

I want to show "Population" of town as label where "Source" is 'Wikipedia' for example.

How can I use "Population" and "Source" attributes from "Towns" expression builder?

Update 1

I've meant getting information only from records where "Source" is 'Wikipedia'

Answer

I've found out a solution.

I've made rule-based filtering with expression:

relation_aggregate( 'TownsInformationRelation', 'max', CASE WHEN "Source" = 'Wikipedia' THEN "Population" ELSE 0 END )

And also I've used filter:

relation_aggregate( 'TownsInformationRelation', 'count', CASE WHEN "Source" = 'Wikipedia' THEN 'true' ELSE NULL END )

Thanks!