code: def query(self):
          
          q = {}
          for field, value in self._pos.items():
              if value is None:
                  q.update({field: {'$exists': True}})
              else:
                  q.update({field: {'$gt': value}})
          return q
docs: A mongdb query expression to find all records with higher values
          for this mark's fields in the collection.
  
          :rtype: dict