title: How to query a DocumentRefrence in Firestore?
body: <p>I'm new in firebase Firestore and when i tried to write this code with Firestore I faced a problem.</p>
  
  <pre><code>DatabaseReference mDisplayData = FirebaseDatabase.getInstance().getReference().child("Users").child(postKey);
                  com.google.firebase.database.Query firstQuery = mDisplayData.limitToFirst(20);
                  firstQuery.addListenerForSingleValueEvent(new ValueEventListener() {
                      @Override
                      public void onDataChange(DataSnapshot dataSnapshot) {
                          if (dataSnapshot.exists()){
                              String nameData = dataSnapshot.child("name").getValue().toString();
                              String statusData = dataSnapshot.child("status").getValue().toString();
                              String imageData = dataSnapshot.child("thumb_image").getValue().toString();
  
                              userViewHolder.setDetails(getApplicationContext(),nameData,statusData,imageData);
                          }
                      }
  
                      @Override
                      public void onCancelled(DatabaseError databaseError) {}
                  });
  </code></pre>
  
  <p>the problem was in this line of code : </p>
  
  <pre><code> Query firstQuery = firebaseFirestore.collection("Users").document(postKey).collection(null);
  </code></pre>
  
  <p>what i want to do is to create a firestore query but i want to stop only in postkey refrence the thing that is not possible what should i do.
  please help and thank you.</p>