class Publication < ActiveRecord::Base
  belongs_to :category
  has_one    :site, :through => :category
  named_scope :on_site,        lambda {|s| {:include => [:site], :conditions => ['sites.slug != ?', 's']}}
end
class Category
  belongs_to :site
  has_many   :publications
end
class Site
  has_many :categories
  has_many :publications, :through => :categories, :foreign_key => 'category_id'
end

Mysql::Error: Unknown column 'categories.category_id' in 'on clause': SELECT
`publications`.`id` AS t0_r0, `publications`.`shoot_id` AS t0_r1,
`publications`.`category_id` AS t0_r2, `publications`.`title` AS t0_r3,
`publications`.`slug` AS t0_r4, `publications`.`publish_on` AS t0_r5,
`publications`.`created_at` AS t0_r6, `publications`.`updated_at` AS t0_r7,
`publications`.`description` AS t0_r8, `publications`.`media_base_path` AS t0_r9,
`sites`.`id` AS t1_r0, `sites`.`name` AS t1_r1, `sites`.`created_at` AS t1_r2,
`sites`.`updated_at` AS t1_r3, `sites`.`slug` AS t1_r4, `sites`.`description` AS t1_r5,
`sites`.`dhd_merch_id` AS t1_r6, `sites`.`members_area_url` AS t1_r7 FROM `publications`
 LEFT OUTER JOIN `categories` ON (`publications`.`id` = `categories`.`category_id`)
 LEFT OUTER JOIN `sites` ON (`sites`.`id` = `categories`.`site_id`) WHERE (sites.slug != 's')

LEFT OUTER JOIN `categories` ON (`publications`.`id` = `categories`.`category_id`)

has_many :publications, :through => :categories, :foreign_key => 'category_id'

class Publication < ActiveRecord::Base
  belongs_to :category
  #has_one    :site, :through => :category
  named_scope :on_site,        lambda {|s| {:include => { :category => :site }, :conditions =>  ['sites.slug != ?', s]}}
end

named_scope :on_site,        lambda {|s| {:joins =>
   ['LEFT OUTER JOIN `categories` ON  (`publications`.`category_id` = `categories`.`id`) ',
   'LEFT OUTER JOIN `sites`      ON  (`sites`.`id` = `categories`.`site_id`)'],
   :conditions => ['sites.slug = ?', s]}}