#30124: QuerySet.only() on reverse many-to-one relationship causes extra
database
queries
-------------------------------------+-------------------------------------
Reporter: Beda | Owner: nobody
Kosata |
Type: Bug | Status: new
Component: Database | Version: 2.1
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
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.
--
Ticket URL: <https://code.djangoproject.com/ticket/30124>
Django <https://code.djangoproject.com/>
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