Literals in foreign key definitions

Hello all,

I was trying something on my 8.3 server that's a bit controversial, and I wasn't surprised it didn't work. I think it would be nice if it were possible though...

The case at hand is that I have a table:

CREATE TABLE unitclass (
name TEXT NOT NULL PRIMARY KEY,
is_baseclass BOOLEAN NOT NULL DEFAULT true
);

That has a many-to-many relationship with:

CREATE TABLE unitclass_relation (
base text NOT NULL,
exponent int NOT NULL,
derived text NOT NULL,

FOREIGN KEY (base, true) REFERENCES unitclass (name, is_baseclass),
FOREIGN KEY (derived, false) REFERENCES unitclass(name, is_baseclass)
);

This does give an error (not unexpected):
ERROR: syntax error at or near "true"
LINE 8: FOREIGN KEY (base, true) REFERENCES unitclass (name...

Now the intent here is to restrict foreign keys referencing the base class to unitclass records that describe a baseclass and to restrict foreign keys referencing a derived class to unitclass records that do NOT describe a baseclass.
Basically I'm trying to disallow derived classes to be derived of other derived classes.

I can of course add a few triggers to force that constraint, but I think it would be nice if the above syntax could be made to work. Or is this already in 8.4 or 8.5 or is this a can of worms? Does the SQL spec disallow it?

Cheers,
Alban Hertroys

--
If you can't see the forest for the trees,
cut the trees and you'll see there is no forest.

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