What is the best way to get a filtered list of docs in root collection, along with all its child docs, that are stored in the subcollection?

Ok, I can get a list of root collection docs, what next: should I query every doc subcollection items separately?

I can do that (looks not optimal), but how my original query function should return a list, how can I get it if all sub-docs have already red (all async queries are complete)?

Thanks.