title: Firebase Firestore add data without overwrite
body: <p>I am trying to add data to my firestore database. The firestore is this:</p>
  
  <p>Firestore db</p>
  
  <p><a href="https://i.stack.imgur.com/XiZB3.png" rel="noreferrer"><img src="https://i.stack.imgur.com/XiZB3.png" alt="enter image description here"></a></p>
  
  <p>I have my php file that loads data into it with this code <code>db.collection('events').doc('documentNameUsingUniqueEventID').set({id1:data1, id2:data2, id3:data3, ...});</code></p>
  
  <p>but when I load new data and old still exists the documents gets overwrite and I lose the changes I made. Is there a way to add data avoiding overwrite existing ones?</p>