simple django custom aggregation

custom_aggregation.py

from django.db.models import Aggregate
from django.db.models.sql import aggregates


def custom_aggregation(select_query):
    class SqlAggregate(aggregates.Aggregate):
        sql_function = ''
        sql_template = select_query

    class VisitorRate(Aggregate):
        sql = SqlAggregate
        def add_to_query(self, query, alias, col, source, is_summary):
            aggregate = self.sql(col,
                                 source=source,
                                 is_summary=is_summary,
                                 **self.extra)
            query.aggregates[alias] = aggregate

    return VisitorRate