I'm still confused on what associations have responsibility for
holding the keys in their own tables and what ones depend on other
tables.  Is this right?

Association               Responsibility
-----------               --------------
has_one                   foreign
has_many                  foreign
belongs_to                local
has_and_belongs_to_many   foreign (in separate table)

So basically, anything with belongs_to should have the key of the
thing it belongs to in its table.  All other associations don't. 
Right?  Have I missed any associations?

Also, in the examples I see some occurrences of "belong_to".  Is this a typo?

Thanks,
Carl