Search by foreign key of foreign key

Hi,

So far I can sort by foreign key.

@section_pages, @sections = paginate :sections, :per_page => 20, :order
=> sort, :include => :title

But how do I sort by a foreign key of the foreign key.

I have “sections” which belong to “courses” which belong to “subjects”.

I want to sort by “prefixes” which is in “subjects”.

Thanks