resolve_aggregate

#!/usr/bin/env python
from django.db.models.aggregates import Aggregate
from django.db.models.sql.aggregates import Aggregate as SqlAggregate
from django.db.models.sql.query import Query


class SqlGetter(SqlAggregate):
    sql_function = ''

class SqlQuery(Query):
    def resolve_aggregate(self, value, aggregate, **kwargs):
        # resolve_aggregate expects value to be a number.
        if isinstance(aggregate, SqlGetter):
            return value
        return super(SqlQuery, self).resolve_aggregate(value, aggregate, **kwargs)

class Getter(Aggregate):
    @property
    def default_alias(self):
        return self.lookup

    def add_to_query(self, query, alias, col, source, is_summary):
        query.aggregates[alias] = SqlGetter(col, source=source, is_summary=is_summary, **self.extra)
        query.__class__ = SqlQuery # Monkey Patch



#How to use it:
from django.db import models


class Related(models.Model):
    some_field = models.CharField()
    field_i_dont_want_to_load = models.TextField()
    field_i_dont_want_to_load2 = models.TextField()
    field_i_dont_want_to_load3 = models.TextField()

class Example(models.Model):
    related = models.ForeignKey(Related)


Example.objects.all().annotate(Getter('related__some_field'))'
# This is similar to:
Example.objects.all().select_related().defer('related__field_i_dont_want_to_load', 'related__field_i_dont_want_to_load2', 'related__field_i_dont_want_to_load3')