[Rails] has_many with :dependent => :destroy not destroying

I have three models: Book, User, and BookOwnership.  Book and User form an m--to-n relationship, but because I will have additional information about the ownership relationships, I cannot use has_and_belongs_to_many, so I've defined the relationships as follows:

class Book < ActiveRecord::Base

  has_many  :book_ownerships, :dependent => :destroy

  has_many  :owners, :through => :book_ownerships, :class_name => "User"

end

class User < ActiveRecord::Base

  has_many  :book_ownerships, :dependent => :destroy

  has_many  :books, :through => :book_ownerships

end

class BookOwnership < ActiveRecord::Base

  belongs_to  :book

  belongs_to  :owner, :class_name => "User"

end

Based on this, I expect that when I call #destroy on an instance of either Book or User it should also destroy the associated BookOwnership instances.  But this doesn't seem to be happening.  Both of the tests below are currently failing as are similar attempts in console.

class BookOwnershipTest < ActiveSupport::TestCase

  context "for a book and a user" do

    setup do

      @book1 = Factory(:book)

      @user1 = Factory(:user)

    end

    context "with an ownership relationship" do

      setup { @oship1 = BookOwnership.create(:book => @book1, :owner => @user1) }

      should "destroy associated BookOwnerships when Book is destroyed" do

        @book1.destroy

        assert(@oship1.destroyed?)

      end

      should "destroy associated BookOwnerships when Owner is destroyed" do

        @user1.destroy

        assert(@oship1.destroyed?)

      end

    end

end

Any advice as to what I might do differently here to get the desired behavior?  Am I using this wrong, or is it a bug in Rails?  (Using version 3.0.5.)

Thanks.

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