gistfile1.py

class Venue(models.Model):
    client = models.ForeignKey(TixUser, null=True)

class Event(models.Model):
    venue = models.OneToOneField(Venue, null=True)


Event.objects.filter(venue__client=SomeClient)

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------- VS ------
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class Event(models.Model):
    client = models.ForeignKey(TixUser, null=True)
    venue = models.OneToOneField(Venue, null=True)

Event.objects.filter(client=SomeClient)