title: Firestore update only one field
body: <p>I have a database. The sequence is: collections - document - hashmaps. For example:</p>
  
  <p>users - the name of the collection</p>
  
  <p>users.uid - the name of the document</p>
  
  <p>Hashmap  the document consists of a lot of hashmaps user data 
  Hashmap
  Hashmap
  etc</p>
  
  <p>Hashmap: the name of user is the key and telephone, location etc are the values. I need to update only one field(location) for one username, but can't understand how do this?</p>
  
  <p>I tried the next way (update phone number for alex):</p>
  
  <pre><code>User user = new User();
  user.setPhone(131902331);
  
  Map&lt;String,RealmObject&gt; userMap = new HashMap&lt;&gt;();
  userMap.put("alex",user);
  
            mFirebaseFirestore
                    .collection("users")
                    .document(mFirebaseAuth.getUid())
                    .set(userMap, SetOptions.merge())
                    .addOnSuccessListener(new OnSuccessListener&lt;Void&gt;() {
                        @Override
                        public void onSuccess(Void aVoid) {
                            LOG.info("Success");
                        }
                    })
                    .addOnFailureListener(new OnFailureListener() {
                        @Override
                        public void onFailure(@NonNull Exception e) {
                            LOG.error("Failure "+e.toString());
                        }
                    });
  </code></pre>
  
  <p>What I am doing wrong? </p>