#30482: prefetch_related_objects evaluates lookup.queryset when checking for
invalid prefetch ordering
-------------------------------------+-------------------------------------
Reporter: Iain P. | Owner: (none)
Watts |
Type: | Status: assigned
Uncategorized |
Component: Database | Version: 2.2
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 |
-------------------------------------+-------------------------------------
When encountering a prefetch it has already seen and checking whether to
throw the "lookup was already seen with a different queryset" ValueError,
prefetch_related_objects will inadvertently evaluate lookup.queryset. For
example:
{{{
foo = Foo.objects.create()
prefetches = [
'bar',
Prefetch('bar', queryset=Bar.objects.filter(my_field=True))
]
prefetch_related_objects([foo], *prefetches)
}}}
Right before it throws the ValueError, the above code will execute the
full unfiltered query `Bar.objects.filter(my_field=True)`. This may match
a very large number of rows (or a whole table), since it is not restricted
to `Bars` related to the specific `foo`. In our case this led to the query
timing out and the page returning a 504 without the code ever actually
reaching the ValueError.
--
Ticket URL: <https://code.djangoproject.com/ticket/30482>
Django <https://code.djangoproject.com/>
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