Hi Jason,
On Nov/07/2020, Jason Johns wrote: > Does https://code.djangoproject.com/ticket/22553 cover this case? It was > closed as a documentation update with this commit in the docs: > https://github.com/django/django/pull/2974/files Thanks very much for the links, it's a related case one could say but not my case. It says "If the data in the database might have changed, you can get updated results for the same query by calling ``all()`` on a previously evaluated ``QuerySet``." It doesn't cover this case because the queryset has never been evaluated yet in my case and the first time that the queryset is evaluated is using a book.id that doesn't exist anymore. The original code: ``` author = Author.objects.create(name='James') book = Book.objects.create(title='test') book.authors.add(author) book.save() authors = book.authors.all() # No queryset evaluation book.delete() # book is deleted from the database print(authors) # First time that the queryset is evaluated # and it uses a book.id that doesn't exist anymore[1] ``` The query launched: SELECT `modeltest_author`.`id`, `modeltest_author`.`name` FROM `modeltest_author` INNER JOIN `modeltest_book_authors` ON (`modeltest_author`.`id` = `modeltest_book_authors`.`author_id`) WHERE `modeltest_book_authors`.`book_id` = 1 LIMIT 21 If using a field as a primary key, deleting the book, creating a new one with the same primary key: it's possible to expect (depends on the user experience) the first set of authors but get the new noe (with the same primary key). Thanks again for the pointers, Carles > > > >From your example, re-doing the book.authors.all() call will refresh the > queryset's cache. > On Wednesday, November 4, 2020 at 5:51:36 AM UTC-5 Carles Pina Estany wrote: > > > > > Hi all, > > > > I have a question about Django ORM. I'd like to confirm that the current > > behaviour is expected (in a way it is!) or find/open an issue if > > needeed. > > > > In this example I've defined two models (see them at the end of the > > email if needed): one for a Book with a ManyToMany to authors, and a > > second model of Author. > > > > And code like this: > > ``` > > author = Author.objects.create(name='James') > > book = Book.objects.create(title='test') > > > > book.authors.add(author) > > book.save() > > > > authors = book.authors.all() > > > > book.delete() > > > > print(authors) # should a user expect all the authors? > > ``` > > > > In the `print(authors)` line the book is already deleted and it returns > > no authors. It executes an SQL query where at that point book.pk does > > not exist. I think that technically it could return wrong existing data > > if the book.pk had been reused, AFAIK none of the default database > > backends would do this but I think that it could happen if doing a field > > with primary_key=True. > > > > If the `authors = book.authors.all()` line was after the > > `book.delete()`: Django raises a ValueError with the error message > > '<Book: test> needs to have a value for field "id" before this > > many-to-many relationship can be used.' > > > > I understand why this happens. In some code I needed something along > > these lines and I'm doing something like "authors = > > list(book.authors.all())" to keep the list of authors, delete the book > > and then do some actions in the authors. > > > > I think that this can be confusing. Also, Django prevents the error in > > one case (get query set after the deletion) but not the other case > > (execute the query after deletion). > > > > Fix idea: > > If QuerySet was holding a book instance it could enforce that book.pk is > > set at the time of running the query (or returning the result if the > > result was cached?) > > > > I had a quick look at QuerySet and the book instance is in > > self._hints['instance']. In `QuerySet._fetch_all` I could do something > > along the lines of: > > ` > > if 'instance' in self._hints and self._hints['instance'].pk is None: > > raise ValueError('Object deleted') > > ` > > > > I guess that in > > create_forward_many_to_many_manager.ManyRelatedManager.__init__ the book > > instance could be saved in some variable in the QuerySet object (I guess > > that self._hints['instance'] is not meant to be used like my example) to > > check this when appropiate. Has this ever been considered? Or is the > > behaviour as it is now good enough? > > > > Thank you very much, > > > > PS: Models used in the example: > > ``` > > class Author(models.Model): > > name = models.TextField(max_length=64) > > > > def __str__(self): > > return self.name > > > > > > class Book(models.Model): > > title = models.TextField(max_length=128) > > authors = models.ManyToManyField(Author) > > > > def __str__(self): > > return self.title > > ``` > > > > -- > > Carles Pina i Estany > > https://carles.pina.cat > > > > -- > You received this message because you are subscribed to the Google Groups > "Django developers (Contributions to Django itself)" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to django-developers+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-developers/28155993-2ea0-4406-b9d0-44da422b3ad5n%40googlegroups.com. -- Carles Pina i Estany https://carles.pina.cat -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-developers+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/20201108112956.GA25509%40pina.cat.
