title: Firestore Reference data type to set Foreign keys
body: <p>By <strong>REFERENCE DATA TYPE</strong> I mean the <a href="https://firebase.google.com/docs/firestore/manage-data/data-types" rel="nofollow noreferrer"><strong>DATA TYPE</strong></a> supported by FireStore. 
   <strong>NOT</strong>  document reference.</p>
  
  <p>How to add a reference data type? </p>
  
  <p>how to use Reference data type as a foreign key which refers to another document?</p>