Is there a way to augment django QuerySets with extra attributes?

Question

I\'m trying to add some extra attributes to the elements of a QuerySet so I can use the extra information in templates, instead of hitting the database multiple times. Let m

Answer 1

Here is my version of alter_items proposed by Will Hardy.

Instead of a single value it allows different values of a custom attribute for each object: you can pass a mapping of values by object id.

It also automatically wraps the results of all methods that return QuerySet.

import types
from itertools import islice

from django.db.models import Model, QuerySet


class QuerySetWithCustomAttributes(object):
    def __init__(self, queryset, **custom_attrs):
        self.queryset = queryset
        self.custom_attrs = custom_attrs

    def __set_custom_attrs(self, obj):
        if isinstance(obj, Model):
            for key, val in self.custom_attrs.items():
                setattr(obj, key, val[obj.id])
        return obj

    def __iter__(self):
        for obj in self.queryset:
            yield self.__set_custom_attrs(obj)

    def __getitem__(self, ndx):
        if type(ndx) is slice:
            return self.__class__(self.queryset.__getitem__(ndx), **self.custom_attrs)
        else:
            return self.__set_custom_attrs(next(islice(self.queryset, ndx, ndx + 1)))

    def __len__(self):
        return len(self.queryset)

    def __apply(self, method):
        def apply(*args, **kwargs):
            result = method(*args, **kwargs)
            if isinstance(result, QuerySet):
                result = self.__class__(result, **self.custom_attrs)
            elif isinstance(result, Model):
                self.__set_custom_attrs(result)
            return result
        return apply

    def __getattr__(self, name):
        attr = getattr(self.queryset, name)
        if isinstance(attr, types.MethodType):
            return self.__apply(attr)
        else:
            return attr