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
30124	QuerySet.only() on reverse many-to-one relationship causes extra database queries	Beda Kosata	Dan Davis	"Using the following example models:

{{{
class WorkSet(models.Model):
    name = models.TextField(blank=True)

class Work(models.Model):
    name = models.TextField(blank=True)
    work_set = models.ForeignKey(WorkSet, on_delete=models.CASCADE, related_name='works')
    extra_data = JSONField(default=dict, blank=True)
}}}

when I iterate over the Works related to a WorkSet {{{ ws }}} like this:

{{{
for work in ws.works.all().only('name'):
    print(work.name)
}}}

Not only one query is made to the database, but 1+N where N is the number of Works related to the WorkSet. In {{{ connection.queries }}} these queries look like this

{{{
'SELECT ""test_work"".""id"", ""test_work"".""work_set_id"" FROM ""test_work"" WHERE ""test_work"".""id"" = 677931'
}}}

etc. That is - the ORM refetches the {{{ work_set_id }}} for each of the Works iterated over. If I change the loop to explicitly include {{{ work_set_id }}}, only one query is run:

{{{
for work in ws.works.all().only('name', 'work_set_id'):
    print(work.name)
}}}

While this behavior makes some sense, I find it quite unexpected and a potential cause of nasty performance 'leaks'. It think that either the documentation should explicitly mention this case and how to work around it or better the ORM should automatically add the relationship key into the fetched fields.

Please let me know if anything is unclear about this issue and if there is anything I can improve. 

"	Bug	closed	Database layer (models, ORM)	2.1	Normal	duplicate			Accepted	0	0	0	0	0	0