[OT?] Derived attributes

Hi folks,

sorry if this might seem a bit too specific to WebObjects: I've used a
couple of derived attributes in my WebObjects app's EOModel. Simple stuff
along the lines of:

(SELECT COUNT(*) FROM address WHERE address.eoid_customer = customer.eoid)

in the Customer entity. (eoid being pk and eoid_customer fk) This worked
nicely with OpenBase, i.e. it simply returned the number of addresses a
particular customer has registered. Using the same expression with Postgres
returns in fact the row count of the address table. Is there a way to make
Postgres use the eoid of the current row instead of checking for matches
between eoid_customer and _any_ eoid in the customer table?

Any hints greatly appreciated!

Fabian