id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
35044	Accessing a deferred field clears reverse relations	Adam Johnson	Giannis Terzopoulos	"`DeferredAttribute.__get__` calls `Model.refresh_from_db()` to load the deferred field. Whilst `refresh_from_db()` does load the intended field, it also clears a lot of cached data, including reverse relations. This can causes extra queries when those relations are accessed before and after a deferred field. 

For example, take these models:

{{{
class Book(models.Model):
    title = models.TextField()
    ...


class BookText(models.Model):
    book = models.OneToOneField(Book, on_delete=models.CASCADE)
    text = models.TextField()
}}}

With query debug logging on, we can see that a second access to `booktext` after accessing the deferred `title` causes an unnecessary extra query:

{{{
In [1]: from example.models import *

In [2]: b=Book.objects.defer('title').earliest('id')
(0.000) SELECT ""example_book"".""id"", ""example_book"".""author_id"" FROM ""example_book"" ORDER BY ""example_book"".""id"" ASC LIMIT 1; args=(); alias=default

In [3]: b.booktext
(0.000) SELECT ""example_booktext"".""id"", ""example_booktext"".""book_id"", ""example_booktext"".""text"" FROM ""example_booktext"" WHERE ""example_booktext"".""book_id"" = 1 LIMIT 21; args=(1,); alias=default
Out[3]: <BookText: BookText object (1)>

In [4]: b.title
(0.000) SELECT ""example_book"".""id"", ""example_book"".""title"" FROM ""example_book"" WHERE ""example_book"".""id"" = 1 LIMIT 21; args=(1,); alias=default
Out[4]: 'A 1'

In [5]: b.booktext
(0.000) SELECT ""example_booktext"".""id"", ""example_booktext"".""book_id"", ""example_booktext"".""text"" FROM ""example_booktext"" WHERE ""example_booktext"".""book_id"" = 1 LIMIT 21; args=(1,); alias=default
Out[5]: <BookText: BookText object (1)>
}}}

This is due to `refresh_from_db()` clearing the reverse-related object cache.

Spotted whilst working on #28586. My implementation for that will probably fix this issue, but I thought it best to report this separately."	Bug	closed	Database layer (models, ORM)	dev	Normal	fixed			Ready for checkin	1	0	0	0	0	0