Django model: fetch last ForeignKey in one query

Question

In Django models, I have two database models like:

A(models.Model):
    ....
    b = models.ForeignKey(B, related_name='my_a')

B(models.Model):
    ....

Now, I would like to fetch all B, but have last A be fetched with the same query.

so something like:

B.objects.all().annotate(myA=fetch last A)

I know for prefetch_related but it is too slow.

how can I do it?