title: Firestore get all docs and subcollections of root collection
body: <p>Say I have this kind of structure</p>
  
  <pre><code>    A (collection): { 
         a (doc): {
             name:'Tim',
             B (collection):{
                 b (doc): {
                        color:'blue'
                 }
               }
            }
      }
  </code></pre>
  
  <p>where <code>A</code> and <code>B</code> are <strong>collections</strong> while <code>a</code> and <code>b</code> are <strong>documents</strong>. <br>
  Is there a way to get everything contained in a root document with one query? <br>
  If I query like this</p>
  
  <pre><code>db.collection("A").doc("a").get()
  </code></pre>
  
  <p>I just gets <code>name:'Tim'</code> field. What I want is to also get all B's documents. <br>
  I basically wish my query returns </p>
  
  <pre><code>         {
             user:'Tim',
             B (collection):{
                 b (doc): {
                        color:'blue'
                 }
               }
            }
  </code></pre>
  
  <p>Is it possibly or do I really need to make multiple queries one for each collection :/ ? <br></p>
  
  <p>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 <code>1 x N</code> where N is the depth of my tree :/.</p>