Why does Django's RelatedManager not cache the object the lookup was called from, on the destination object?

Question

If I have the following models:

class Fubar(models.Model):
    name = models.CharField()

class Related(models.Model):
    fubar = models.ForeignKey(Fubar)

I would expect that the ORM would magically cache the parent Fubar object if I accessed Related using .related_set:

fubar = Fubar.objects.all()[0]
related = fubar.related_set.all()[0]
related.fubar

That results in 3 queries, where I would expect it to only result in 2, since related.fubar could be optimised in this context to be the same object I called a RelatedManager on.

This is fixed in django 1.5 - https://docs.djangoproject.com/en/dev/releases/1.5-beta-1/#caching-of-related-model-instances — Darb, Nov 29 '12 at 20:00

mjtamlyn · Answer 1 · 2012-02-02T15:34:00.077

2

Whilst I'm not sure why this doesn't work (except maybe magic reduction), you could easily avoid the extra query with

fubar.related_set.select_related('fubar')[0]

edited Feb 02 '12 at 15:34

answered Jan 31 '12 at 10:17

mjtamlyn

1,704
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That does help, thanks. I was hoping it would be fubar's exact object/state, since in the current instance I am fetching and caching other information on the fubar object. I will just have to maintain my own cache of objects and lookup using the fubar_id. – Darb Jan 31 '12 at 10:43

score 0 · Answer 2 · answered Jan 31 '12 at 10:18

0

In django 1.4 they are introducing prefetch_related which will automatically retrieve, in a single batch, related objects for each of the specified lookups.

answered Jan 31 '12 at 10:18

Ross

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Why does Django's RelatedManager not cache the object the lookup was called from, on the destination object?

2 Answers2