`only_deleted` doesn't work sometimes(?)

[pboling]

I am not sure what causes this, but here is the symptom of the problem:

[31] pry(main)> p.email_messages.only_deleted.count
=> 0
[32] pry(main)> p.email_messages.with_deleted.count
=> 320
[33] pry(main)> p.email_messages.count
=> 157

I would expect only_deleted + natural = with_deleted. There are truly that number of deleted records, but when using only_deleted the count is 0.

This is my first attempt to use only_deleted, so maybe it doesn't work all the time... Don't know. The relationship isn't a basic has_many, but a has_many :through, and that may be relevant.