help with table constraint / check

hello,
This is my first post to the list, so if I make any mistakes, bear with me. I
have a table that has the follow attributes:

fs_id
integer
not null default
nextval('public.extranet_filesystem_fs_id_seq'::text)
parent_id
integer
not null default 0
fs_name
character varying(255)
not null
fs_type
character varying(4)
not null default 'file'
Indexes
extranet_filesystem_pkey primary key btree (fs_id)
extranet_filesystem_u1 unique btree (parent_id, fs_name)
Check constraints:
"extranet_filesystem_fs_type"
((fs_type = 'file'::character varying) OR (fs_type = 'dir'::character varying))
Foreign Key constraints:
$1 FOREIGN KEY (parent_id) REFERENCES extranet_filesystem(fs_id) ON UPDATE
CASCADE ON DELETE CASCADE

What I want to do is set up a constraint / check so that when a record is added,
it takes the parent_id being passed in the INSERT statement, looks at the record
whose fs_id equals the parent_id, and makes sure that the fs_type for that record
is 'dir' and not 'file'.

Can I do this with standard constraints or do I need to use some PL/pgSQL
features? Anyone have a few hints? Thanks!

-Cliff