Constraint question

I have the following table:

create table onp_crm_businessfield_company(
businessfield_id integer not null references onp_crm_businessfield(id),
company_id integer not null references onp_crm_relation(id),
is_preferred boolean,
UNIQUE(businessfield_id, company_id)
);

I want a constraint on "is_preffered" so that it's only allowed to be set once
pr. businessfield_id pr. company so that only one businessfield can be
preferred for a company. Does anyone have an idea how to enforce this?

--
Andreas Joseph Krogh <andreak(at)officenet(dot)no>
Senior Software Developer / Manager
gpg public_key: http://dev.officenet.no/~andreak/public_key.asc
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