erro de OO

oo.py

class TagGeral(models.Model):
    tag = models.CharField(max_length=50, verbose_name='Tag secundarias')

class Post(models.Model):
    tag = models.ManyToManyField(TagGeral)
    tag_lista = models.CharField(max_length=100, blank=True)

    def get_tags(self):
        tags = self.tag_lista.split(', ')
        tag_geral = TagGeral.objects.all()
        for i in tags:
            if tag_geral.filter(tag=i):
                tag_filtered = tag_geral.get(tag__iexact=i)
                self.tag.add(tag_filtered)