title: firebase/firestore: create and store unique id for documents (users for USERID)
body: <p>I would like to generate unique ID for each user based on the location. eg: if the user is from newyouk, ID would be NY-23234. This should happen the very first time when the user sign up to my app.</p>
  
  <p>I see that I could use auto generated firestore ID - but it is too long for assigning as id and it can't have any other characters with it. I am using angularfire2 to connect to firestore.</p>
  
  <p>What I am thinking now is :
  every time a new user is signed up, save the user details in 'users collection' (with firestore authogenerated key as the document key ) and run a firebase function for item creation in 'users' collection. It should return the last singup users ID from the same location and add '1' to it. then save that number to the newly created user.</p>
  
  <p>any suggestions plz? any other way to do it.</p>