title: Firebase Cloud Firestore : Invalid collection reference. Collection references must have an odd number of segments
body: <p>I have the following code and getting an error : </p>
  
  <pre><code>Invalid collection reference. Collection references must have an odd number of segments
  </code></pre>
  
  <p><strong>And the code :</strong></p>
  
  <pre><code>private void setAdapter() {
          FirebaseFirestore db = FirebaseFirestore.getInstance();
          db.collection("app/users/" + uid + "/notifications").get().addOnCompleteListener(task -&gt; {
              if (task.isSuccessful()) {
                  for (DocumentSnapshot document : task.getResult()) {
                      Log.d("FragmentNotifications", document.getId() + " =&gt; " + document.getData());
                  }
              } else {
                  Log.w("FragmentNotifications", "Error getting notifications.", task.getException());
              }
          });
      }
  </code></pre>