code: def is_valid(self, context, sid):
  		
  		
  		record = self._Document.find_one(sid, project=('expires', ))
  		
  		if not record:
  			return
  		
  		return not record._expired
docs: Identify if the given session ID is currently valid.
  		
  		Return True if valid, False if explicitly invalid, None if unknown.