title: How to increment existing number field in Cloud Firestore
body: <p>I have a field which indicates the number of transactions. I would like to increment it by <code>1</code> as soon as a transaction occurs. I could'nt find any method to directly execute this.</p>
  
  <p>One method is to get the user first and then update the field with the received value <code>+ 1</code>, but this is too lengthy and slow.</p>
  
  <p>Can anyone tell me any other feasible method?</p>