def get_sum(self, field=None):
        """
        Create a sum aggregation object and add it to the aggregation dict

        :param field: the field present in the index that is to be aggregated
        :returns: self, which allows the method to be chainable with the other methods
        """

        if not field:
            raise AttributeError("Please provide field to apply aggregation to!")
        agg = A("sum", field=field)
        self.aggregations['sum_' + field] = agg
        return self