firebase - Firestore get all docs and subcollections of root collection

Question

Say I have this kind of structure

    A (collection): { 
       a (doc): {
           name:'Tim',
           B (collection):{
               b (doc): {
                      color:'blue'
               }
             }
          }
    }

where A and B are collections while a and b are documents.
Is there a way to get everything contained in a root document with one query?
If I query like this

db.collection("A").doc("a").get()

I just gets name:'Tim' field. What I want is to also get all B's documents.
I basically wish my query returns

         {
           user:'Tim',
           B (collection):{
               b (doc): {
                      color:'blue'
               }
             }
          }

Is it possibly or do I really need to make multiple queries one for each collection :/ ?

Say I have a really deep nested tree of collections representing the user profile, my costs will raise like hell since each time I load a user profile I have a multiplier of read requests 1 x N where N is the depth of my tree :/.

Answer 1

Waitting for answers