get_bound_objects

get_bound_objects.py

#!/usr/bin/env python
# coding: utf-8


def get_bound_objects(queryset, classes):
    """
    Helper function to map a list of objects to corresponding classes in Django ORM.

    It is a simple and stupid alternative to django_polymorphic.
    Good to use with custom Paginator class. Example usage:

    class Order(models.Model):
        # some common fields
        pass

    class CashOrder(Order):
        # some specific fields
        pass

    class CreditCardOrder(Order):
        # some specific fields
        pass

    class PaypalOrder(Order):
        # some specific fields
        pass

    class OrderPaginator(Paginator):
        def page(self, number):
            _page = super(OrderPaginator, self).page(number)
            _page.object_list = get_bound_objects(_page.object_list, [CashOrder, CreditCardOrder, PaypalOrder])
            return _page

    class OrdersListView(ListView):
        model = Order
        paginator_class = OrderPaginator
        paginate_by = 20

    """

    ids = tuple(i.id for i in queryset.only('id'))
    objects = [None for i in ids]
    pos_map = dict((obj_id, i) for i, obj_id in enumerate(ids))
    bound = set()

    for cls in classes:
        for obj in cls.objects.filter(id__in=ids):
            obj_id = obj.id
            if obj_id in bound:
                raise ValueError("Multiple objects exists for id %d" % obj_id)
            bound.add(obj_id)
            objects[pos_map[obj_id]] = obj

    unbound = set(ids).difference(bound)
    if unbound:
        for obj in queryset.model.objects.filter(id__in=unbound):
            objects[pos_map[obj.id]] = obj

    return objects