Django 1.2 helper for approximating select_related on a M2M relation

load_related_m2m.py

"""Django 1.2 helper for approximating select_related on a M2M relation

Source: http://softwaremaniacs.org/forum/django/11038/

"""

from django.db.models.sql.constants import LOOKUP_SEP
from django.db.models import sql
from django.db import connections


def load_related_m2m(object_list, field, connection=connections['default']):
    """Retrieve the related objects in a M2M relationship in a single database query.

    Adds a new property all_field to each object in your queryset.

    """
    select_fields = ['pk']
    related_field = object_list.model._meta.get_field(field)
    related_model = related_field.rel.to
    cache_name = 'all_%s' % field

    for f in related_model._meta.local_fields:
        select_fields.append('%s%s%s' % (field, LOOKUP_SEP, f.column))

    query = sql.Query(object_list.model)
    query.add_fields(select_fields)
    query.add_filter(('pk__in', [obj.pk for obj in object_list]))

    related_dict = {}
    for row in query.get_compiler(connection=connection).results_iter():
        if row[2]:
            related_dict.setdefault(row[0], []).append(related_model(*row[1:]))

    for obj in object_list:
        try:
            setattr(obj, cache_name, related_dict[obj.pk])
        except KeyError:
            setattr(obj, cache_name, [])

    return object_list