django Manager on 2. class-level?

Question

I want to write the Manager of a Model into the Model-class itself.

class Post(models.Model):
    title = models.CharField(max_length=50)
    slug = models.SlugField(max_length=50, blank=True, unique=True)
    body = models.TextField(blank=True)
    pub_date = models.DateTimeField(blank=True)
    mod_date = models.DateTimeField(blank=True)
    tags = models.ManyToManyField(Tag)

    class Meta:
        ordering = ['pub_date'] # Newest first

    class Manager(models.Manager):

        def by_slug(self, slug):
            slug = slug.strip().lower()
            return self.get(slug=slug)

    objects = Manager()

    def __unicode__(self):
        return self.title

    def save(self, *args, **kwargs):
        self.slug = self.slug or generate_slug(self.title)
        self.pub_date = self.pub_date or timezone.now()
        self.mod_date = timezone.now()
        return super(Post, self).save(*args, **kwargs)

But I get the following error:

Traceback (most recent call last):
  File "C:\Users\niklas\Desktop\blog_project\manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\core\management\__init__.py", line 443, in execute_from_command_line
    utility.execute()
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\core\management\__init__.py", line 382, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\core\management\base.py", line 196, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\core\management\base.py", line 231, in execute
    self.validate()
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\core\management\base.py", line 266, in validate
    num_errors = get_validation_errors(s, app)
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\core\management\validation.py", line 30, in get_validation_errors
    for (app_name, error) in get_app_errors().items():
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\db\models\loading.py", line 158, in get_app_errors
    self._populate()
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\db\models\loading.py", line 64, in _populate
    self.load_app(app_name, True)
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\db\models\loading.py", line 88, in load_app
    models = import_module('.models', app_name)
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\utils\importlib.py", line 35, in import_module
    __import__(name)
  File "C:\Users\niklas\Desktop\blog_project\blog\models.py", line 48, in <module>
    class Post(models.Model):
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\db\models\base.py", line 99, in __new__
    new_class.add_to_class(obj_name, obj)
  File "C:\Python27\lib\site-packages\django-1.4.1-py2.7.egg\django\db\models\base.py", line 219, in add_to_class
    value.contribute_to_class(cls, name)
TypeError: Error when calling the metaclass bases
    unbound method contribute_to_class() must be called with Manager instance as first argument (got ModelBase instance instead)

Can you tell me why this error occurs and if what I want to do is even possible?

Answer 1

It looks like you can't, this is a bug in Django, you can report it :)