Custom Django aggregate function for replacing average value of NULL with 0.

custom_aggregate.py

import django.db.models.sql.aggregates
import django.db.models.aggregates

class AvgWithZeroForNull(django.db.models.sql.aggregates.Avg):
    sql_template = 'COALESCE(%(function)s(%(field)s), 0)'
django.db.models.sql.aggregates.AvgWithZeroForNull = AvgWithZeroForNull

class AvgWithZeroForNull(django.db.models.aggregates.Avg):
    name = 'AvgWithZeroForNull'
django.db.models.aggregates.AvgWithZeroForNull = AvgWithZeroForNull

# Usage (to retrieve objects with highest average, NULLs become zeroes and are last):
# MyModel.objects.annotate(average=AvgWithZeroForNull('other_model__field_name')).order_by('-average')