Polymorphic associations. Agile rails book confusion.

Nope - looks like you've got it. To clear up your initial confusion,
when they say "can't arrange to have the foreign key point back to all
three tables", think of it as "can't arrange to have *one single*
foreign key point back to all three tables". Of course you can do it
with three foreign keys, but as you now understand, that isn't the
point of polymorphic associations.