title: How to add values to Firebase Firestore without overwriting?
body: <p>I have two Activities, I am adding data to Firestore from these two activities individually. But, whenever I add second activity data to Firestore, it is overwriting the first activity data. I used below code in the two activities:</p>
  
  <pre><code> firebaseFirestore.collection("Users").document(user_id).set(data)
  </code></pre>
  
  <p>How to stop overwriting? I want to save both Activities data in the same <code>user_id</code>.</p>