:joins is ignored in a find within an habtm association extension

Hi,
I've added some stuff to /web/rails/activerecord/test/cases/associations/habtm_join_table_test.rb

I've attached a full diff, but here's the interesting bit.

class MyReader < ActiveRecord::Base                                                                                                                   
  has_and_belongs_to_many :my_books do                                                                                                                
    def unreviewed_books                                                                                                                              
      self.find(:all,                                                                                                                                 
      :joins => "left join my_book_reviews on my_books.my_book_id = my_book_reviews.my_book_id",                                                      
      :conditions => "my_book_reviews.my_book_id is null")                                                                                            
    end                                                                                                                                               
  end                                                                                                                                                 
end

When the unreviewed_books function is called, a query is built that incorporates the :conditions, but not the :joins -

SELECT * FROM "my_books" INNER JOIN "my_books_my_readers" ON "my_books".id = "my_books_my_readers".my_book_id WHERE ("my_books_my_readers".my_reader_id = 1 AND (my_book_reviews.my_book_id is null))

Which then fails with missing FROM-clause entry for table "my_book_reviews"

Anyone else come across this or have any ideas?