#28939: QuerySet used by prefetch_related() does not use expected connection.
-------------------------------------+-------------------------------------
Reporter: Nick Pope | Owner: nobody
Type: Bug | Status: new
Component: Database | Version: master
layer (models, ORM) | Keywords: prefetch,
Severity: Normal | prefetch_related, using, connection
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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I've run into an case with prefetching where the connection used for the
prefetch queries is not the one I would expect:
{{{#!python
# When using the default database connection:
In [1]: from django.db import connections
In [2]: connections['default'].queries
Out[2]: []
In [3]: connections['readonly'].queries
Out[3]: []
In [4]: User.objects.prefetch_related('operators').first()
Out[4]: <User: example>
In [5]: connections['default'].queries
Out[5]:
[{'sql': 'SELECT ... FROM "auth_user" ORDER BY "auth_user"."id" ASC LIMIT
1', 'time': '0.205'},
{'sql': 'SELECT ... AS "_prefetch_related_val_user_id", ... FROM
"operator" INNER JOIN "operator_users" ON ("operator"."id" =
"operator_users"."operator_id") WHERE "operator_users"."user_id" IN (1)
ORDER BY "operator"."code" ASC', 'time': '0.011'}]
In [6]: connections['readonly'].queries
Out[6]: []
# When specifying the database connection to use:
In [1]: from django.db import connections
In [2]: connections['default'].queries
Out[2]: []
In [3]: connections['readonly'].queries
Out[3]: []
In [4]:
User.objects.using('readonly').prefetch_related('operators').first()
Out[4]: <User: example>
In [5]: connections['default'].queries
Out[5]:
[{'sql': 'SELECT ... AS "_prefetch_related_val_user_id", ... FROM
"operator" INNER JOIN "operator_users" ON ("operator"."id" =
"operator_users"."operator_id") WHERE "operator_users"."user_id" IN (1)
ORDER BY "operator"."code" ASC', 'time': '0.010'}]
In [6]: connections['readonly'].queries
Out[6]:
[{'sql': 'SELECT ... FROM "auth_user" ORDER BY "auth_user"."id" ASC LIMIT
1', 'time': '0.002'}]
}}}
In the second case I would have expected all queries to be executed on the
{{{readonly}}} connection by default.
This can be achieved by using {{{Prefetch('operators',
queryset=Operator.objects.using('readonly'))}}}, but it means that plain
strings cannot be used and often a lot of nested {{{Prefetch()}}} objects
can be required.
A solution to this could be to do the following:
1. Use the connection from the original {{{QuerySet}}} that called
{{{.prefetch_related()}}} by default.
2. ~~Possibly add {{{QuerySet.prefetch_related(using=...)}}} to allow
overriding for all prefetch queries.~~ (Doesn't fit nicely with the API.)
3. Possibly add {{{Prefetch(using=...)}}} to allow overriding for a
specific branch of prefetching. (This would propagate down.)
--
Ticket URL: <https://code.djangoproject.com/ticket/28939>
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