Need help modeling complex entity relationships

I'm attempting to create a simple web app that will allow users to do
sentence completion exercises. We have users, stems (first part of
sentences to be completed), stem-sets (a particular subset/collection
of stems), and stem responses. It looks like the primary
entities/tables/associations that I need are:

USERS
has_and_belongs_to_many STEM_SETS

STEMS
has_and_belongs_to_many STEM_SETS

STEM_SETS
has_and_belongs_to_many USERS
has_and_belongs_to_many STEMS

STEM_RESPONSES
??

Not sure about which relationships the stem responses model should
have. It seems like they should belong to the combination of USERS,
STEMS and STEM_SETS.

If I were just creating the db table for this, I would have:

STEM_RESPONSES
id
user_id
stem_set_id
stem_id
text

Is there a way to represent this with Rails relationships? Or am I
missing a much easier way to do this?