title: FIRESTORE - Insert a subCollection inside a document
body: <p>I have a collection named Reports in my Firestore database, I've already done adding documents inside it. But my problem now is I want to add a Document with a SubCollection inside Reports, any Idea?<br></p>
  
  <pre><code>public CollectionReference Ref = firestore.collection("Reports");
  Reports reports = new Reports(date, cash, total, discount, quantity, sum, employee);
  Ref.add(reports);
  </code></pre>