Rails - Oct 2009 - Proper Way to Force Model to be Creation-time Only Member of another

Hi all,

What is the proper way to do this.  I have a Bundle model.  And I wish
for it to be able to have many Products.  But I wish that the products
that a bundle has only be able to be set at the creation time of the
Bundle.  That is once a Bundle is created I should not be able to
remove or add Products to it.

thanks,
Aaron