Not In Relation SELECT

Hi all,

i need to load from DB all model objects wich aren't invested by a relation. For example, if MODEL A hasMany MODEL B, how can i retrieve all MODEL B wich are not related to MODEL A instance?

Symfony provides whereNotIn method to invert the relation during select for this purpose and i need something similar. Without PDO support for this i'm currently using PHQL to retrieve these objects but i feel PDO is lacking a common required feature here.

Thanks