Updating sub-attirbutes without selecting the parent

I am working on an application and need to be able to update a single
field of nested record in the applications. For instance,

Library has_many Books
Book has_many Chapters

I would like to be able to have a single form on the Library page that
allows me to use a pull down list that selects the book and the chapter
and have users be able to update that chapters’ associated fields (mark
the chapter as updated for instance). It doesn’t need to be smart and
pull the existing values for the chapter record. I would be happy to
use form_tag and make it a dumb form to just dump the data into the
database but I am not sure how to best approach it. I have tried using
the nested form_for and fields_for methods but they appear to break
after one “layer” of nesting. Lets assume all chapters have a key field
that is unique across the whole chapters table; is there a way I could
create a form that would ignore associations and just look for that key
field and update that record - ignoring that the chapter is owned by a
book and the book is owned by a library? Thanks,