expression to find out null relationship

[Marcin Skladaniec]

Hi

I have a simple one-to-many relationship : AppleTree - Apple. I'm  
doing a query :
new SelectQuery(AppleTree.class, expression)
how should the expression look like to return the trees with apples ?
expression = ExpressionFactory.noMatchExp(AppleTree.APPLES, null)  
does not seem to work, neither does
expression = ExpressionFactory.noMatchExp("applesArray", null)
Regards
Marcin