Can I annotate on a OneToMany relationship for performance?

Question

I have the following simplified models.

class Service(models.Model):
    ...

class ServiceLogs(models.Model):
    service = models.ForeignKey(Service, related_name='logs', ...)
    who_did = models.ForeignKey(User, ...)
    role_played = models.CharField(choices=(('CA','CA'),
                                            ('CO', 'CO'),
                                            ('TO', 'TO')), ...)

I know I can find out who played what role by calling <service obj>.logs.filter(role='CA'), but this approach hits the database 3 times per object given I have 3 roles.

I was reading the documentation about aggretation and annotation, but all the examples use numeric values like Sum, Avg, etc. I tried using Q for lookups, but Django complains with the message 'WhereNode' object has no attribute 'output_field'

Can I make this query more efficient so I hit the database once per object?