I have a use-case where I am querying an array of ObjectIDs. For example:
I'd like the results to be ordered by `_id.$in`. Right now I'm resorting to client-side sorting, which kind of sucks since it requires converting between ObjectIDs and strings among other hacks.
A special sorting option like this would be awesome (similar to $natural):
Although the attribute being `$in`ed need not be `_id`, it should be unique (and thus indexed).
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