code: def where(self, field_path, op_string, value):
          
          query = query_mod.Query(self)
          return query.where(field_path, op_string, value)
docs: Create a "where" query with this collection as parent.
  
          See
          :meth:`~.firestore_v1beta1.query.Query.where` for
          more information on this method.
  
          Args:
              field_path (str): A field path (``.``-delimited list of
                  field names) for the field to filter on.
              op_string (str): A comparison operation in the form of a string.
                  Acceptable values are ``<``, ``<=``, ``==``, ``>=``
                  and ``>``.
              value (Any): The value to compare the field against in the filter.
                  If ``value`` is :data:`None` or a NaN, then ``==`` is the only
                  allowed operation.
  
          Returns:
              ~.firestore_v1beta1.query.Query: A filtered query.