Fixed #10589 -- Removed duplicated model definition in null_fk_ordering...

    def __unicode__(self):

        return self.comment_text

class Comment(models.Model):

    post = models.ForeignKey(Post, null=True)

    comment_text = models.CharField(max_length=250)

    class Meta:

        ordering = ['post__forum__system_info__system_name', 'comment_text']

    def __unicode__(self):

        return self.comment_text

__test__ = {'API_TESTS': """

# Regression test for #7512 -- ordering across nullable Foreign Keys shouldn't