Djangocon EU: auto-prefetching with model field fetch modes in Django 6.1 - Jacob Walls¶

Tags: django, djangocon

(One of my summaries of the 2026 Djangocon EU in Athens).

There’s an example to experiment with here: https://dryorm.xterm.info/fetch-modes-simple

Timeline: it will be included in Django 6.1 in August.

The reason is the 1+n problem:

books = Book.objects.all()
for book in books:
    print(book.author.name)
    # This does a fresh query for author every time.

You can solve it with select_related(relation_names) or prefetch_related(relation_names). The first does an inner join. The second does two queries.

But: you might miss a relation. You might specify too many relations, getting data you don’t need. Or you might not know about the relation as the code is in a totally different part of the code.

Fetch mode is intended to solve it. You can append .fetch_mode(models.FETCH_xyz) to your query:

• models.FETCH_ONE: the current behaviour, which will be the default.

• models.FETCH_PEERS: Fetch a deferred field for all instances that came from the same queryset. More or less prefetch_related in an automatic, lazy manner.

• models.FETCH_RAISE: useful for development, it will raise FieldFetchBlocked. And it will thus tell you that you’ll have a performance problem and that you might need FETCH_PEERS

This is what happens:

books = Book.objects.all().fetch_mode(models.FETCH_PEERS)
for book in books:
    # We're iterating over the query, so the query executes and grabs all books.
    print(book.author.name)
    # We accessed a relation, so at this point the prefetch_related-like
    # mechanism ist fired off and all authors linked to by the books are
    # grabbed in one single query.

You can write your own fetch modes, for instance if you only want a warning instead of raising an error.

Unrelated photo explanation: a cat I encountered in Athens on an evening stroll in the neighbourhood behind the hotel.