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
36157	Unusual behaviour when pre-fetching with only applied on the related fields	Tim McCurrach		"When prefetching related models. If you apply `.only()` to the related queryset, django performs additional lookups for related IDs that have been left out of the `only`.

It is probably easiest to explain the issue with an example. Suppose you have these models:

{{{
class Blog(models.Model):
    name = models.CharField(max_length=100)

class Post(models.Model):
    name = models.CharField(max_length=100)
    blog = models.ForeignKey(Blog, on_delete=models.CASCADE, related_name=""posts"")
    ...lots of other big fields
}}}
And you create a few items for each:
{{{
blog = Blog.objects.create(name=""Django Tricks"")
blog2 = Blog.objects.create(name=""React Tricks"")
Post.objects.create(name=""prefetching"", blog=blog)
Post.objects.create(name=""models"", blog=blog)
Post.objects.create(name=""templates"", blog=blog)
Post.objects.create(name=""hooks"", blog=blog2)
Post.objects.create(name=""components"", blog=blog2)
}}}
If I wish to pre-fetch the posts for some blogs, but only want the names of each post, rather than the content of each post I can do the following:
{{{
Blog.objects.prefetch_related(Prefetch(""posts"", queryset=Post.objects.only(""name"")))
}}}
I would expect this to result in just 2 database queries. One to fetch the data for the `Blog` instances, and another to fetch the the data for the related `Post`s. Instead, there is an n+1 issue where there are 5 extra follow up requests for each of the related `Post` instances. This is the SQL that is generated:
{{{
SELECT ""app_blog"".""id"", ""app_blog"".""name"" FROM ""app_blog"" LIMIT 21
SELECT ""app_post"".""id"", ""app_post"".""name"" FROM ""app_post"" WHERE ""app_post"".""blog_id"" IN (1, 2)
SELECT ""app_post"".""id"", ""app_post"".""blog_id"" FROM ""app_post"" WHERE ""app_post"".""id"" = 1 LIMIT 21
SELECT ""app_post"".""id"", ""app_post"".""blog_id"" FROM ""app_post"" WHERE ""app_post"".""id"" = 2 LIMIT 21
SELECT ""app_post"".""id"", ""app_post"".""blog_id"" FROM ""app_post"" WHERE ""app_post"".""id"" = 3 LIMIT 21
SELECT ""app_post"".""id"", ""app_post"".""blog_id"" FROM ""app_post"" WHERE ""app_post"".""id"" = 4 LIMIT 21
SELECT ""app_post"".""id"", ""app_post"".""blog_id"" FROM ""app_post"" WHERE ""app_post"".""id"" = 5 LIMIT 21
}}}
I can understand it might be a good idea to have the related-id's for the blog on hand should you need them later. But I also think, that by using `.only()` you are explicitly telling django - I don't need these. This is a real problem for larger data-sets, where you end up with thousands of extra round-trips to the database.

"	Bug	new	Uncategorized	5.1	Normal			Tim McCurrach	Unreviewed	0	0	0	0	0	0