Cache the generic relation field of all the objects in the queryset, using larger bulk queries ahead of time. Improved from original by Daniel Roseman: http://blog.roseman.org.uk/2010/02/22/django-patterns-part-4-forwards-generic-relations/ and justinfx's gist cache_generics.py : https://gist.github.com/justinfx/3095246#file-cache_generics-py Su…

generic_relations.py

'''
Cache the generic relation field of all the objects 
in the queryset, using larger bulk queries ahead of time.
 
Improved from original by Daniel Roseman:
http://blog.roseman.org.uk/2010/02/22/django-patterns-part-4-forwards-generic-relations/

and

justinfx's gist cache_generics.py :
https://gist.github.com/justinfx/3095246#file-cache_generics-py

Supports customized object_id_field and GenericForeignKey name.

'''

from operator import attrgetter
from django.contrib.contenttypes.models import ContentType
 
def cache_generic_content_types(queryset, object_id_field='object_id', content_type_fk='content_object'):
    """
    Django does not support select_related on generic foreign key. Thus some
    ORM actions may trigger N+M querys(N is item number of the queryset and
    M is the item number of the content type). This function will cache content
    and reduce N+M querys to 2 querys.

    object_id_field: see https://docs.djangoproject.com/en/1.3/ref/contrib/contenttypes/#django.contrib.contenttypes.generic.GenericRelation
    content_type_fk: the name of GenericForeignKey which linked to content type
    """
    get_object_id = attrgetter(object_id_field)
    generics = {}
    for item in queryset:
        if get_object_id(item) is not None:
            generics.setdefault(item.content_type_id, set()).add(get_object_id(item))
 
    content_types = ContentType.objects.in_bulk(generics.keys())
 
    relations = {}
    for ct, fk_list in generics.iteritems():
        ct_model = content_types[ct].model_class()
        relations[ct] = ct_model.objects.in_bulk(list(fk_list))
 
    for item in queryset:
        try:
            cached_val = relations[item.content_type_id][get_object_id(item)]
        except KeyError:
            cached_val = None
        setattr(item, '_%s_cache' % content_type_fk, cached_val)
    return queryset