Tracker #28633: Update GraphQL to work with Rails 6
Graphql association preload doesn't work with Rails 6
Rails 6 is changing behaviour of preloading associations.
It no longer assigns customly preloaded associations to the record itself and thus such associations doesn't seem preloaded.
Preloaded records are then ignored on subsequent calls to record.<association_name>.
We use custom preloading, so we need to update accordingly.