title: Firebase Swift - How to create a child and add its id to another ref property?
body: <p>As described <a href="https://stackoverflow.com/questions/38868947/firebase-database-structure-need-advice">here</a>, I'd like to store Book objects in a separate ref and store its id value inside <code>books</code> property of <code>User</code></p>
  
  <pre><code>Users:
  user_id:121jhg12h12
    email: "john@doe.com"
    name: "John Doe"
    profile_pic_path: "https://...".
    language: "en"
    exp_points: 1284
    friends: [user_id]
    books: [[book_id, status, current_page, start_date, finish_date]]
    badges: [[badge_id, get_date]]
  
  Books:
  book_id: 3213jhg21
    title: "For whom the bell tolls"
    author: "Ernest Hemingway"
    language: "en"
    pages_count: 690
    ISBN: "21hjg1"
    year: 2007
  </code></pre>
  
  <p>Whenever I add a book inside the app</p>
  
  <pre><code>self.ref!.child("Books").childByAutoId().setValue(["title": arrayOfNames[0] as! NSString, "author": arrayOfNames[1] as! NSString , "pages_count":arrayOfNames[2] as! NSString])
  </code></pre>
  
  <p>The book object is created in Books ref, but I'd like to add its id to the User's books array of user right away. </p>
  
  <p>Can it be done in some nifty way, instead of querying the book, retrieve its ID and add it to the array? </p>
  
  <p>If no, what's the proper way o querying the object id that has been just created?</p>
  
  <p>Maybe I should not use AutoId mode and create unique Id for each object by myself in app?</p>