#24254: Passing a query to "pk__in" of another query may result in invalid SQL
-------------------------------------+-------------------------------------
Reporter: bronger | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.7
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | 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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Changes (by inglesp):
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
Comment:
Have had a little dig into this.
Consider an `__in` lookup with the following on the RHS:
`M.objects.order_by(sort_field).distinct()[m:n]`.
This generates a query that returns both `M`'s primary key and
`sort_field`. Because it returns two fields, it cannot be used as a
subquery (at least in SQLite) and so we hit the reported error.
I think this behaviour is related to a
[https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.distinct
warning in the docs]: "Any fields used in an `order_by()` call are
included in the SQL `SELECT` columns" (although from reading the code for
`SQLCompiler.get_extra_select` I think that the fields used in an
`order_by()` call are only included in the `SELECT` columns when the query
has `distinct` set).
It's not obvious to me why any fields used in an `order_by()` call
'''must''' always be included in the `SELECT` columns, so I'd like to
propose a fix whereby `SQLCompiler.get_extra_select` returns an empty list
if it's dealing with a subquery. I've made that change locally, and
nothing fails. However, I don't know enough about the ORM to know whether
this is a good idea! I hope this is useful.
--
Ticket URL: <https://code.djangoproject.com/ticket/24254#comment:1>
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