Django: Queryset.clear() method on OneToOneField()

I need to avoid cascading deletes on a foreign key, but it's a OneToOneField(), like:

class MyModel(models.Model):
    def delete(self):
        self.mysubmodel.clear()  # Breaks because self.cartitem is not a QuerySet.
        super(MyModel, self).delete()

class MySubModel(models.Model):
    mymodel = models.OneToOneField(MyModel)

This version would work (but I can't use this version):

class MyModel(models.Model):
    def delete(self):
        self.mysub开发者_开发百科model_set.clear()  # Works because self.mysubmodel_set is a QuerySet.
        super(MyModel, self).delete()

class MySubModel(models.Model):
    mymodel = models.ForeignKey(MyModel)

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Setting the field to None in the delete method should work:

self.mysubmodel = None

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What is expected behavior here? I mean, o2o does not allow null values?