title: Sort Firestore documents by a user defined hierarchy?
body: <p>I have a Firestore db and it has some documents including a field for "position".
  This "position" field can take values "senior lecturer", "Head of department", "Lecturer".
  I want to sort this collection such that, the documents containing "Head of department" as the position should come to the top, the documents containing "senior lecturer" as the position should come next and the documents containing "lecturer" as the position should come to the bottom.</p>
  
  <p>I can't just sort this alphabetically. ("H", "S" ,"L")</p>
  
  <p>How do I do this in Firestore?</p>