prefetch

class prefetch.Prefetcher(filter=None, reverse_mapper=None, decorator=None, mapper=None, collect=None)

Prefetch definitition. For convenience you can either subclass this and define the methods on the subclass or just pass the functions to the contructor.

Eg, subclassing:

class GroupPrefetcher(Prefetcher):

    @staticmethod
    def filter(ids):
        return User.groups.through.objects.filter(user__in=ids).select_related('group')

    @staticmethod
    def reverse_mapper(user_group_association):
        return [user_group_association.user_id]

    @staticmethod
    def decorator(user, user_group_associations=()):
        setattr(user, 'prefetched_groups', [i.group for i in user_group_associations])

Or with contructor:

Prefetcher(
    filter = lambda ids: User.groups.through.objects.filter(user__in=ids).select_related('group'),
    reverse_mapper = lambda user_group_association: [user_group_association.user_id],
    decorator = lambda user, user_group_associations=(): setattr(user, 'prefetched_groups', [
        i.group for i in user_group_associations
    ])
)

Glossary:

• filter(list_of_ids):

  A function that returns a queryset containing all the related data for a given list of keys. Takes a list of ids as argument.

• reverse_mapper(related_object):

  A function that takes the related object as argument and returns a list of keys that maps that related object to the objects in the queryset.

• mapper(object):

  Optional (defaults to lambda obj: obj.id).

  A function that returns the key for a given object in your query set.

• decorator(object, list_of_related_objects):

  A function that will save the related data on each of your objects in your queryset. Takes the object and a list of related objects as arguments. Note that you should not override existing attributes on the model instance here.