#33682: Clarify using distinct() with related fields that have Meta.ordering
defined.
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Reporter: Robert Leach | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Documentation | Version: 3.2
Severity: Normal | Resolution:
Keywords: sql, distinct | Triage Stage: Accepted
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 Robert Leach):
That's definitely better, as it directly references Meta.ordering, which
is a great clue to what the implications are. What do you think about
**adding** something like:
> If all you want is to make a set of query results distinct without
changing the ordering, note you must explicitly "re-"add the otherwise
over-ridden fields defined in Meta.ordering. But be careful, if you
simply add those fields, you can run afoul of the matching fields
requirement between order_by and distinct. The field(s) defined in
Meta.ordering can include a foreign key, which will resolve differently in
order_by (to the related model's Meta.ordering field(s)) and distinct (the
`_id`) and the fields will no longer match between the 2 expressions.
>
> But this gotcha can be entirely avoided by only ever using real database
fields in every `Meta.ordering`, `order_by`, and `distinct` list instead
of adding Django's related model objects (e.g. `ForeignKey`,
`ManyToManyField`, `OneToManyField`, etc.) so as not to rely on Django's
differing related-model-object to database-field resolving mechanisms
(e.g. instead of adding `blog` to `Entry.Meta.ordering`, add
`blog__name`).
...but like I said, all the info is there to either put it together and
anticipate it (albeit requiring a fairly deep understanding and more
experience to achieve those insights), or to interpret the exception. I
just feel like piecing together this (what I would consider common) use-
case, may make it clearer to those who are endeavoring into their first
big Django project and applying complex understandings of and experiences
with other programming languages.
Whatever edit gets included, I would like to say that I very much
appreciate your attention to this topic.
--
Ticket URL: <https://code.djangoproject.com/ticket/33682#comment:9>
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