#26481: Add a "strict mode" for defer()/only()
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Reporter: adamchainz | Owner: nobody
Type: New feature | Status: new
Component: Database layer (models, ORM) | Version: master
Severity: Normal | Keywords: only, defer
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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In our project at YPlan we had some highly optimized queries using
`only()` to restrict the fields being fetched.
Some time later the downstream code of these queries was changed to access
extra fields not in the `only()` list. This triggered the
`DeferredAttribute` implicit queries to go fill in those fields with
single queries, a classic N+1 queries problem.
Whilst better testing on the queries would have revealed this, I can't
help but feel that it's a somewhat surprising behaviour that `only()` (and
by extension, `defer()`) can bite back this badly.
I'd like some way of making `only()` and `defer()` strict, and to raise an
exception on deferred fields.
Currently we've implemented this with a `patchy.patch` to insert a `raise`
into `DeferredAttribute`:
{{{
from django.db.models.query_utils import DeferredAttribute
patchy.patch(DeferredAttribute.__get__, """\
@@ -20,6 +20,7 @@ def __get__(self, instance, owner):
# might be able to reuse the already loaded value. Refs
#18343.
val = self._check_parent_chain(instance, name)
if val is None:
+ raise AttributeError("Deferred field '{}' fetched from
database!".format(self.field_name))
instance.refresh_from_db(fields=[self.field_name])
val = getattr(instance, self.field_name)
data[self.field_name] = val
""")
}}}
I can imagine this being a Django-wide setting, or maybe per-model so it
wouldn't affect third party apps so badly.
--
Ticket URL: <https://code.djangoproject.com/ticket/26481>
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