title: Using provider UID as Firestore ID in AngularFire2 auth
body: <p>I want to add Google, Facebook and custom login to my Angular7 web app. </p>
  
  <p>Is the UID identifier given by each provider unique ? I mean, <strong>can I use it as a user ID in my users Firestore collection</strong> ?</p>
  
  <p>And if that's the case, can I still use the <code>createId()</code> method to generate this ID myself in my custom login ? Or might it be the same as one of the providers UIDs ?</p>