class MyResource(ModelResource):

    def obj_update(self, bundle, request=None, **kwargs):
        for field_name in self.fields:
            field = self.fields[field_name]
if type(field) is ToOneField and field.null and bundle.data[field_name
    ] is None:
    setattr(bundle.obj, field_name, None)
return super(MyResource, self).obj_update(bundle, **kwargs)
Использование TastyPie для обновления поля ForeignKey до нуля