We recently hit an issue where Review Bot was unable to find any diffs

on newly-created review requests. This ended up being due to stale query

caches caused by select_related() and prefetch_related() queries from

ReviewRequestResource.get_queryset(), which would get called by

ReviewRequestDraftResource.update() prior to publishing.
Those caches were intended to reduce queries when requesting a list of

resources, but interfered with operations when working with single

items. In this case, it resulted in any signal handlers for the

published signals being unable to query for change descriptions or

diffsets using relations from the review request, returning only stale

results. While the diffset bug is relatively new, the change

descriptions bug has been around a long while, and simply hasn't been

hit in production yet.
This change updates the queryset calculation to only select/prefetch

related objects when returning a list of items.
A unit test was added that reliably reproduces our Review Bot failure

case (along with a failure involving change description query caches).

Unit tests failed before the fix with the same failure case experienced

with Review Bot. After the fix, unit tests pass.