EOQualifier on a toMany attribute

Hi,
I don't know if I'm doing it wrong or if I'm just misuse EOQualifier  
but:
I have an entity a with a flattened manyToMany relationship named  
connectors on a connector entity.

I need to qualify a fetch on entity a based on having connector %@
so basically I was doing
EOQualifer.qualifierWithQualifierFormat("connectors = %@", arg);
arg having a connector entity.

But this doesn't seemed to work...
Am I doing this wrong?

Thanks

Xavier