#32549: Add `Q.empty()` to check for nested empty Q objects like `Q(Q())`
-------------------------------------+-------------------------------------
Reporter: jonathan-golorry | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: Q objects, nested, | Triage Stage:
empty | Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by jonathan-golorry):
* status: closed => new
* resolution: wontfix =>
Old description:
> Empty Q objects (`Q()` or `~Q()`, etc) evaluate to `False`, but `Q(Q())`
> evaluates to `True`. This interferes with the shortcut on logical
> operations involving empty Q objects and can lead to bugs when trying to
> detect empty operations.
> {{{
> def q_any(iterable):
> q = Q()
> for element in iterable:
> q |= element
> if q:
> return q
> return Q(pk__in=[])
> Item.objects.filter(q_any()) # no items
> Item.objects.filter(q_any([Q()])) # no items
> Item.objects.filter(q_any([Q(Q())])) # all items
> }}}
>
> Patch https://github.com/django/django/pull/14127 removes empty Q objects
> from `args` during Q object initialization.
>
> This requires https://code.djangoproject.com/ticket/32548 in order to
> prevent a regression in logical operations between query expressions and
> empty Q objects.
New description:
Empty Q objects (`Q()` or `~Q()`, etc) evaluate to `False`, but `Q(Q())`
evaluates to `True`. This interferes with the shortcut on logical
operations involving empty Q objects and can lead to bugs when trying to
detect empty operations.
{{{
def q_any(iterable):
q = Q()
for element in iterable:
q |= element
if q:
return q
return Q(pk__in=[])
Item.objects.filter(q_any()) # no items
Item.objects.filter(q_any([Q()])) # no items
Item.objects.filter(q_any([Q(Q())])) # all items
}}}
~~Patch https://github.com/django/django/pull/14127 removes empty Q
objects from `args` during Q object initialization.~~
Patch https://github.com/django/django/pull/14130 adds `.empty()` for
detecting nested empty Q objects and uses that instead of boolean
evaluation for shortcutting logical operations between empty Q objects. So
`Q(x=1) | Q(Q())` will now shortcut to `Q(x=1)` the same way `Q(x=1) |
Q()` does. No simplifying is done during Q object initialization.
This requires https://code.djangoproject.com/ticket/32548 in order to
prevent a regression in logical operations between query expressions and
empty Q objects.
--
Comment:
Here is an alternate approach that adds a way to check for nested empty Q
objects instead of making them impossible to create.
--
Ticket URL: <https://code.djangoproject.com/ticket/32549#comment:3>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
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