Reload through_record that has been destroyed in create_through_record

[kamipo]

This is an alternative of #27714.

If has_one :through association has set nil, through_record is
destroyed but still remain loaded target in through_proxy until
reload or reset explicitly.

If through_proxy is not reset (remain destroyed (frozen) target),
setting new record causes RuntimeError: Can't modify frozen hash.

To prevent RuntimeError, should reload through_record that has been
destroyed in create_through_record.

[rafaelfranca]

Why this alternative is better than reseting the proxy?

[kamipo]

I tried to fix stale_state? to regard as destroyed target is staled.
But some tests expects destroyed target is not staled (doesn't expect reload or reset destroyed target immediately).

rails/activerecord/test/cases/nested_attributes_test.rb

Line 356 in 6d8fdd1

assert @pirate.ship.destroyed?

This means that the reloading first is a breaking change. So I changed to reloading on demand.

[rafaelfranca]

Makes sense. Thansk