## db/migrate/product_proxy.rb

ProxyMigration < Something
  def self.up
    create_table :product_proxies do |t|
      t.belongs_to :product # product_id
      t.belongs_to :manufacturer # if of the Manufacturer object
      t.integer :alternate_id # the id the manufacturer gives it.
    end
  end

  def self.down
    drop_table :product_proxies
  end
end

## app/models/product.rb

class Product < ActiveRecord::Base
  has_one :product_proxy
  has_one :manufacturer, :through => :product_proxy
end

## app/models/product_proxy.rb

class ProductProxy < ActiveRecord::Base
  belongs_to :product
  belongs_to :manufacturer

  named_scope :alternate do |product|
    { :conditions => { :product => product } }
  end
end