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on read_mixin.rb:45 Kasket unconditionally does find(missing_ids). find obviously raises when an id is not present. This is correct for a call like 'User.find([1,2,3])' (where user #3 is destroyed), but something like User.all(:include => :group) will raise when a User references a destroyed group object.
The "correct" rails semantics here are to leave the association unfilled.