#30124: QuerySet.only() on reverse many-to-one relationship causes extra
database
queries
-------------------------------------+-------------------------------------
Reporter: Beda Kosata | Owner: Dan Davis
Type: Bug | Status: assigned
Component: Database layer | Version: 2.1
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Dan Davis):
There seem to be two general approaches here:
=== only is intended for fields of the base table
So, if the alias_map has cardinality > 1, then detect fields that are not
from the base table and raise a warning. Since only is an optimization,
the warning alerts the user to the problem, but his/her webapp simply
continues to work, albeit slower. In a reverse relationship, the
parent's key is a field of the base table - add it automatically. This
doesn't necessarily mean that the pk of the parent table will be added.
If the ForeignKey is on something other than the parent pk, then the
parent pk is not what is added to the response. This would fix this
issue, #30124, and in the case of #23051, a warning would be raised and
the call to only would not modify the query. Since only is an
optimization, this may be acceptable.
=== only is not intended for joins
So, if the table_map has cardinality > 1, then raise some warning so that
we can gain feedback from users, and if reasonable, deprecate at a later
time. The code in #30124 would have to be done with defer() or values(),
which could in a real-world case be a lot of fields and a great loss of
utility. #23051 would similarly cause a warning, but here the warning
might be better received ;).
Are there implications of this on other query types, such as
union/difference, etc.?
--
Ticket URL: <https://code.djangoproject.com/ticket/30124#comment:4>
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