#25787: Add existence checks for related database sets on managers
-------------------------------------+-------------------------------------
Reporter: JustusAdam | Owner:
| andersonresende
Type: New feature | Status: closed
Component: Database layer | Version: 1.8
(models, ORM) |
Severity: Normal | Resolution: wontfix
Keywords: ManyToMany, | Triage Stage:
contains, in | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
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Comment (by JustusAdam):
I understand but I think this should be explained in the documentation
rather than using the inefficient way straight away.
Any (normal) user, that does not dig too deep into the workings of
QuerySets will, use {{{in}}} instead of {{{filter().exists()}}} because
that's the python way of doing it. I don't think it is a good idea to
condemn them to potentially very inefficient queries for an operation that
could be much cheaper.
The actual behaviour of {{{__contains__}}} could be explained in the
documentation which someone very worried about efficiency can read and
then use {{{list()}}} to force evaluation if they need a reproducible
query.
{{{filter().exists()}}} poses very little overhead because it queries on
an indexed field that I don't think we'd have to worry about the overhead.
It is not like the current implementation doesn't have its pitfalls. Using
{{{list()}}} on an evaluated QuerySet won't execute queries unlike on en
unevaluated QuerySet. What I am trying to say is that operations on
QuerySet's ''already'' do not work the same way every time, that's simply
their nature. As I understand the mission of django it is to abstract away
complexity and deliver efficiency, which I believe this change would
bring.
Theoretically even using {{{in}}} on an evaluated query set is potentially
very inefficient if the set is large, since it is stored in a list rather
than a {{{set}}}.
--
Ticket URL: <https://code.djangoproject.com/ticket/25787#comment:7>
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