Rails - Dec 2005 - Limit user ability to edit subset of data?

Hi all,

I have different types of user permissions on a site. Some users are 
able to edit specific pages, some users have ''editor'' rights,
and some
have ''admin'' rights.

I can render different views depending on the permissions the user has, 
and restrict the ability to edit certain subsets of data through a form. 
But how can I protect a user (who has permission to access the
''edit''
action) from submitting a raw POST request to edit specific fields that 
they don''t have permission to edit?

Thanks,

Tom