Creating form for associated has_many model

This example shows how to create a form for an associated model 'Comment', where 'Comment' belongs_to 'Post' and 'Post has_many 'Comments'.

How might I modify that to display all the comments in the form, and still have it call the CommentsController (versus having the form call the PostsController, as in the example here #17 HABTM Checkboxes - RailsCasts)?

Thanks

To ask another way:

Using a Polymorphic Assocation, if Photo and Article each 'has_many' Comments, the comment form should call the CommentsController, as shown here #154 Polymorphic Association - RailsCasts -- But what if I'm editing multiple comments in one form for a given Photo. In this case should the form still call the CommentsController or is it better off calling the PhotoController?