Make Django models comparable based on the Meta.ordering

comparablemodel.py

class ComparableModel(object):
    """
    Make models comparable based on the Meta.ordering. Usage:

    >>> import datetime
    >>> class MyModel(ComparableModel, models.Model):
    >>>     name = models.CharField(max_length=255)
    >>>     birthdate = models.DateField()
    >>>     class Meta:
    >>>         app_label = 'test'
    >>>         ordering = ['name', '-birthdate']
    >>> bob = MyModel(name='Bob', birthdate=datetime.date(1970, 1, 1))
    >>> bob2 = MyModel(name='Bob', birthdate=datetime.date(2001, 1, 1))
    >>> igor = MyModel(name='Igor', birthdate=datetime.date(2010, 1, 1))
    >>> bob2 < bob < igor
    True
    """
    def __cmp__(self, other):
        def cmp_tuple(field):
            if field.startswith('-'):
                return getattr(other, field[1:]), getattr(self, field[1:])
            else:
                return getattr(self, field), getattr(other, field)

        # build up a list of two-tuples of values from both models, based on
        # the Meta.ordering
        #
        # comparables = [(self.category, other.category), (self.title, other.title)]
        comparables = map(cmp_tuple, self._meta.ordering)
        # flatten the list of two-tuples into two lists and pass that to cmp
        #
        # cmp([self.category, self.name], [other.category, other.name])
        return cmp(*zip(*comparables))