update.py

from django.db.models.query import CollectedObjects

def update_related_field(obj, value, field):
    """
    Set `field` to `value` for all objects related to `obj`.
    Based on delete object code of `CollectedObjects`:
    http://code.djangoproject.com/browser/django/trunk/django/db/models/query.py
    """
    # Collect all related objects.
    collected_objs = CollectedObjects()
    obj._collect_sub_objects(collected_objs)
    classes = collected_objs.keys()
    # Bulk update the objects for performance
    for cls in classes:
        items = collected_objs[cls].items()
        pk_list = [pk for pk, instance in items]
        cls._default_manager.filter(id__in=pk_list).update(**{field:value})
    return obj