Rails core - Dec 2011 - 3.2rc1 through association with dynamic conditions broken when using joins/includes

My app has feeds and items. Users can subscribe to feeds, which I track in 
a Subscription class.

I wanted an association for all of a user''s unread items:

class Subscription < ActiveRecord::Base
  belongs_to :feed
  belongs_to :user
  has_many :unread_items, :through => :feed, :source => :items,
:conditions
=> proc {
    "not exists ( select * from readings where readings.item_id = items.id 
AND readings.user_id = #{user_id} )"
  }

...

This works fine if I reference it directly (i.e, 
@user.subscriptions.first.unread_items). But if I try to eager load or join 
against it:

eager load: ''self'' in the conditions proc refers to the
intermediate object
(here, the Feed class) which I don''t think is expected. Crashes because
Feed obviously has no user_id in my app.
join:  ''self'' in the conditions proc refers to a
JoinDependency object,
which seems totally wrong. 

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Er, to clarify, the error I''m getting is that the interpolation of 
#{user_id} in the association condition is undefined. 

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