Kasket doesn't understand the difference between .find() and :include => 

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.
