QuerySet used by prefetch_related() does not use expected connection.

[Nick Pope]

I've run into an case with prefetching where the connection used for the prefetch queries is not the one I would expect:

# 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.)