checking data integrity in a recursive table

Hi, I have a table that references itself to create a tree-like structure,
eg:
CREATE TABLE tree (
id SERIAL NOT NULL,
name VARCHAR(255) NOT NULL,
parent INT NULL,
customer IN NOT NULL,
CONSTRAINT parent_key...
CONSTRAINT customer_fk FOREIGN KEY (customer) REFERENCES customer
);
ALTER TABLE tree ADD CONSTRAINT FOREIGN KEY (parent) REFERENCES tree;

As you can see tree also references the customer table.

What I need is a CHECK that will ensuer that any given "tree" row has the
same customer as its parent.
Remember that "parent" can also be NULL.

Or would this be better done as a trigger?

TIA!

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Luke Pascoe
KMG (NZ) Limited. http://www.kmg.co.nz
Mobile: (021) 303019
Email: luke(dot)p(at)kmg(dot)co(dot)nz
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