Django: How to use OuterRef() for .get() in annotate?

Question

I am using django-treebeard and use a model that allows recursive relations.

So, I need to count the number of my descendants using .annotate(), but To get it, you need to go through .get() once, as shown below.

However, in that case, OuterRef("pk") does not seem to work and the following error occurs. This queryset contains a reference to an outer query and may only be used in a subquery.

What can I do to solve this problem?

class Comment(MP_Node):
    post = models.ForeignKey(Post, on_delete=models.CASCADE)
    text = models.TextField( max_length=1000)
    commented_at = models.DateTimeField(default=timezone.now)
    parent = models.ForeignKey(
        "self",
        blank=True,
        null=True,
        on_delete=models.CASCADE,
        related_name="replies",
    )

queryset.filter(post=post)
.annotate(
    reply_count=Count(
        Comment.objects.get(
            pk=OuterRef("pk")
        ).get_descendants()
    )
)