#32704: QuerySet.defer() doesn't clear deferred field when chaining with only().
-------------------------------------+-------------------------------------
Reporter: Manuel Baclet | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 3.1
(models, ORM) |
Severity: Normal | Resolution:
Keywords: defer only | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Mariusz Felisiak):
* stage: Unreviewed => Accepted
Comment:
Replying to [ticket:32704 Manuel Baclet]:
> Considering a simple `Company` model with four fields: `id`, `name`,
`trade_number` and `country`. If we evaluate a queryset containing a
`.defer()` following a `.only()`, the generated sql query selects
unexpected fields. For example:
>
> {{{#!python
> Company.objects.only("name").defer("name")
> }}}
> loads all the fields with the following query:
> {{{#!sql
> SELECT "company"."id", "company"."name", "company"."trade_number",
"company"."country" FROM "company"
> }}}
This is an expected behavior, `defer()` removes fields from the list of
fields specified by the `only()` method (i.e. list of fields that should
not be deferred). In this example `only()` adds `name` to the list,
`defer()` removes `name` from the list, so you have empty lists and all
fields will be loaded. It is also
[https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.only
documented]:
{{{
# Final result is that everything except "headline" is deferred.
Entry.objects.only("headline", "body").defer("body")
}}}
> {{{#!python
> Company.objects.only("name").defer("name").defer("country")
> }}}
> also loads all the fields with the same query:
> {{{#!sql
> SELECT "company"."id", "company"."name", "company"."trade_number",
"company"."country" FROM "company"
> }}}
I agree you shouldn't get all field, but only `pk`, `name`, and
`trade_number`:
{{{
SELECT "ticket_32704_company"."id", "ticket_32704_company"."name",
"ticket_32704_company"."trade_number" FROM "ticket_32704_company"
}}}
this is due to the fact that `defer()` doesn't clear the list of deferred
field when chaining with `only()`. I attached a proposed patch.
--
Ticket URL: <https://code.djangoproject.com/ticket/32704#comment:2>
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