Django: Difference between save() and create()

    # the version that runs fine
    def add_tag(self, obj, tag_name):
        """
        Associates the given object with a tag.
        """
        tag_names = parse_tag_input(tag_name)
        if not len(tag_names):
            raise AttributeError(_('No tags were given: "%s".') % tag_name)
        if len(tag_names) > 1:
            raise AttributeError(_('Multiple tags were given: "%s".') % tag_name)
        tag_name = tag_names[0]
        try:
            tag = Tag.objects.get(slug=slugify(tag_name))
        except Tag.DoesNotExist:
            tag = Tag(name=tag_name)
            tag.save()
        else:
            # check if there is a preferred synonym for this tag
            try:
                related_tag = RelatedTag.objects.get(tag=tag, relation_type='=>')
            except RelatedTag.DoesNotExist:
                pass
            else:
                # there is a preferred synonym; use it instead of the original
                tag = related_tag.related_tag
        ctype = ContentType.objects.get_for_model(obj)
        try:
            ti = TaggedItem.objects.get(tag=tag, content_type=ctype, object_id=obj.pk)
        except TaggedItem.DoesNotExist:
            pass
        else:
            ti = TaggedItem(tag=tag, content_type=ctype, object_id=obj.pk)
            ti.save()


    # the one that raises db error
    def add_tag(self, obj, tag_name):
        """
        Associates the given object with a tag.
        """
        tag_names = parse_tag_input(tag_name)
        if not len(tag_names):
            raise AttributeError(_('No tags were given: "%s".') % tag_name)
        if len(tag_names) > 1:
            raise AttributeError(_('Multiple tags were given: "%s".') % tag_name)
        tag_name = tag_names[0]
        if tagging_settings.FORCE_LOWERCASE_TAGS:
            tag_name = tag_name.lower()
        tag, created = self.get_or_create(slug=slugify(tag_name),
                                          defaults={'name': tag_name})
        if not created:
            # check if there is a preferred synonym for this tag
            try:
                related_tag = RelatedTag.objects.get(tag=tag, relation_type='=>')
            except RelatedTag.DoesNotExist:
                pass
            else:
                # there is a preferred synonym; use it instead of the original
                tag = related_tag.related_tag
        ctype = ContentType.objects.get_for_model(obj)
        TaggedItem._default_manager.get_or_create(
            tag=tag, content_type=ctype, object_id=obj.pk)