How to prevent multiple inserts of same item (by some criteria)

Hi,

Is there a way to prevent JPA/OpenJPA from doing multiple inserts?

I have a somewhat inconvenient database design, whereby some types held in
a table only allow a single instance to be created (other types do allow
it). This makes preventing multiple instances not so easy to achieve with
simple unique constraints.

What I effectively want to do is something like this:

INSERT INTO Applications (..., type)
VALUES (..., someType)
WHERE NOT EXISTS (SELECT FROM Applications WHERE type = someType)

That is for some Applications where only a single instance is allowed for a
type, to prevent multiple instances from being inserted.

Only answer I have found so far is:

http://www.quora.com/Relational-Databases/JPA-conditional-insertion-RDBMS-transactions-isolation

But I cannot make sense of it.

Thanks for your help.

Rupert