Clarification on Django’s .all() Behavior with QuerySets and Prefetched Data

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I’m encountering conflicting information in the Django documentation regarding the .all() method. According to the QuerySet docs (https://docs.djangoproject.com/en/5.1/ref/models/querysets/#django.db.models.query.QuerySet.all):

When a QuerySet is evaluated, it typically caches its results. If the data in the database might have changed since a QuerySet was evaluated, you can get updated results for the same query by calling all() on a previously evaluated QuerySet.

But in the prefetch_related section, using .all() on sub-objects appears to use the cached results:

PYTHON
>>> restaurants = Restaurant.objects.prefetch_related(
...     Prefetch("pizzas", queryset=queryset),
... )
>>> vegetarian_pizzas = restaurants[0].pizzas.all()

This example shows that vegetarian_pizzas doesn’t trigger a new database query.

So, which is it? What does .all() actually do in these different contexts?

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

The key is that the behavior of .all() depends on context and evaluation state. Here’s what’s happening:

.all() on a normal QuerySet:
Calling .all() on a QuerySet creates a new QuerySet that “copies” the current one. If the original QuerySet has already been evaluated (i.e., its results are cached), then calling .all() will also use that cache. If it hasn’t been evaluated, then later evaluation of the new QuerySet will trigger the database query.
Additionally, the Django docs mention that if you want to re-run the query on an already evaluated QuerySet (to catch database changes), you can call .all() on it, which returns a new QuerySet that isn’t using the old cache unless it’s in a context that relies on caching.
.all() on a related manager provided by prefetch_related:
When you use prefetch_related, Django runs a separate query and then “attaches” the set of related objects to the parent instance, storing them in a cache (typically in _prefetched_objects_cache). When you later call something like restaurants[0].pizzas.all(), you aren’t hitting the database because the related manager has already been populated with the prefetched results. Its .all() method simply returns the cached results.

So, the apparent conflict is just that .all() is context sensitive:

On an ordinary QuerySet, evaluation happens per its caching rules.
On a related-manager (especially one lazily populated by prefetch_related), .all() returns the already fetched (and cached) objects.

This design avoids unnecessary database queries for already-populated relationships while still letting you re-run or copy a QuerySet for fresh evaluation in other cases.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by OrbitalAstronaut048 • 1 month ago

Here, apparently vegetarian_pizzas doesn't trigger a database query.

QuerySets itself don't trigger queries. QuerySets are lazy, this means that only if you enumerate, call str(…) or len(…) on it, or anything else that actually needs the data it will make a query.

A QuerySet also caches the result once a query is made, such that enumerate over it a second time, will not query the database a second time.

Now .all() [Django-doc] makes a clone of the QuerySet, and that clone thus has a different cache, this thus means that if you call .all() and then you evaluate the original queryset, it will cache the results in the original queryset, but not in its .all() clone. So:

PYTHON
qs1 = my_queryset
qs2 = qs1.all()
print(qs1)
print(qs2)

will make two queries.

The confusion probably stems from the fact that .all() also has an extra use-case when you are working with a manager. You can not enumerate over a manager, like my_restaurant.pizzas, it thus is used as a tool to get the underlying QuerySet by using .all(). Indeed, if we look at the source code [GitHub]:

PYTHON
    def all(self):
        # We can't proxy this method through the `QuerySet` like we do for the
        # rest of the `QuerySet` methods. This is because `QuerySet.all()`
        # works by creating a

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