Referential integrity (foreign keys) across multiple tables

Simplified schema:

create table hosts (
id serial primary key,
hostname text not null
);

create table pages (
id serial primary key,
hostid int not null references hosts (id),
url text not null,
unique (hostid, url)
);

create table page_contents (
pageid int not null references pages (id),
section text not null
);

(There are many hosts, many pages per host, and many page_contents
sections per page).

Now I want to add a column to page_contents, say called link_name,
which is going to reference the pages.url column for the particular
host that this page belongs to.

Something like:

alter table page_contents add link_name text;
alter table page_contents
add constraint foo foreign key (p.hostid, link_name)
references pages (hostid, url)
where p.id = pageid;

Obviously that second statement isn't going to compile.

I don't want to add the hostid column to page_contents table because I
have a lot of old code accessing the database which would be hard to
change (the old code would no longer be able to insert page_contents
rows).

Is this possible somehow? Perhaps by adding a second table? Do I
have to use triggers, and if so is that as robust as referential
integrity?

Rich.

--
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com