Belongs_to without referencing an id as a foreign key?

Hello everyone,

Here’s my problem for the day: I have three tables that look a little
like
the following

table_a
id

table_b
id
position
table_a_id
name

table_c
id
table_a_id
table_b_position

There will be many rows in table_c that have the same table_a_id, and
the
same table_b_position. These will all be referencing the same name
value in
table_b. The row in table_b may or may not exist, and table_b_position
is a
meaningful field with or without a corresponding row in table_b, so
these
two reasons keep me from using table_b_id within table_c. The natural
relationship here is for table_c to belong_to table_b, but I do not know
how
to do this. Any ideas?

Thanks!
Zack