#29527: Multi-column comparisons
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Reporter: Ryan Hiebert | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 2.0
Severity: Normal | Resolution:
Keywords: QuerySet.extra | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Ryan Hiebert):
With the help of a patient colleague (thanks Tim!) I've been able to
accomplish what I needed to natively, without using {{{.extra()}}}. With
one caveat, I needed to add the LHS as a calculated column using annotate,
which is a bit of a kludge, which ends up returning annotated data that I
didn't really need. For my case, that's not too big of a deal, but perhaps
less than ideal.
It might be deserving of a way to remove annotations that don't need to be
selected, or to allow filter expressions that allow a dynamic expression
on both sides. I'm not sure if that deserves its own ticket, if it should
be on this one, or if perhaps there's already a ticket open for that.
Here's what we came up with:
{{{#!python
from django.db.models import Func, F, Subquery
def after(queryset, value):
fields = ['a', 'b', 'c', 'id']
return (
queryset
.annotate(rank=Func(*(F(field) for field in fields),
function='ROW'))
.filter(rank=Subquery(queryset.filter(id=value).values(*fields)))
.order_by(*fields)
)
}}}
One other thing to note is that this opts to use the explicit {{{ROW}}}
function, rather than relying on that being the result of just using
parenthesis, because parenthesis alone with a single value doesn't
necessarily make a {{{ROW}}}.
--
Ticket URL: <https://code.djangoproject.com/ticket/29527#comment:1>
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