Query on value of foreign key

Question

I have two classes: Plan and InvestmentOption. Plan has a Collection of InvestmentOptions. In the DB, the INVESTMENT_OPTION table has a PLAN_OID column which is a foreign key to PLAN. PLAN's primary key is called OID.
Now suppose I have a Plan OID and I want to retrieve all InvestmentOptions within that Plan identified by the OID, without retrieving the Plan object. Here's the code:
int planOID = 1;
Expression expr = new ExpressionBuilder().get("plan").get("oid").equal(planOID);
session.readAllObjects(InvestmentOption.class, expr);
TopLink executes the following query:
SELECT t1.oid, t1.name, t1.plan_oid FROM plan t0
, investment_option t1 WHERE ((t0.oid = 1) AND (t0.oid = t1.plan_oid))
TopLink joins PLAN and INVESTMENT_OPTION even though that's not necessary - it would have been sufficient to say:
SELECT t1.oid, t1.name, t1.plan_oid FROM investment_option t1 WHERE (1 = t1.plan_oid)
One way to avoid the join is to add a plan_oid attribute to InvestmentOption, map INVESTMENT_OPTION.PLAN_OID to plan_oid as a read-only attribute, and modify the expression as follows:
Expression expr = new ExpressionBuilder().get("plan_oid").equal(planOID);
Is there another way to accomplish this without adding the read-only attribute?

Answer 1

You can define a query key[1] 'planId' on InvestmentOption for the PLAN_OID column. That'll let you write an expression like the one below which does not produce a join. The nice part about the query key is that it does not require you to add attributes to your object in order to query o them.
Expression plan = new ExpressionBuilder();
Expression where = plan.get("planId").equal(somePlanId);--Shaun
[1] http://www.oracle.com/technology/products/ias/toplink/doc/10131/main/_html/descfg010.htm#sthref3039