Querying for no related object on n-m relationship

Question

Hi,
I have a n-m relationship between two objects A and B. I want to query all A objects that do not have a B (a.getB().isEmpty()). Is there a way to build an expression for that ?
I cannot use getField("B_FK").isNull as the foreign key is in a relationship table and I'd like to avoid using direct SQL, so I'm kind of stuck.
Thank you for any help you could provide me.
Bye.

Answer 1

The trick is to use a nested query, and think of the relationship in the other direction (i.e., not that A has many B's, but that B belongs to an A).
There is an example of something similar to this on page 1-32 of the 903 "Using TopLink Foundation Library Guide". The example shows a query to find all the Employees who manage more than 5 employees.
Here is a similar example I wrote recently to find all the HockeyTeams without any Players on them (sorry, it's NHL playoff time and I am Canadian.
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ExpressionBuilder team = new ExpressionBuilder();
ExpressionBuilder player = new ExpressionBuilder();
ReportQuery subQuery =new ReportQuery(Player.class, player);
subQuery.addCount();
subQuery.setSelectionCriteria(player.get("team").equal(team));
Expression exp = team.subQuery(subQuery).equal(0);
ReadAllQuery q = new ReadAllQuery(HockeyTeam.class, exp);
Vector v = (Vector)serverSession.acquireClientSession().executeQuery(q);