Preload unique related object in Django

Question

This works just fine and only performs one DB query:

qs = Blog.objects.filter(post=Subquery(
    Post.objects.filter(blog=OuterRef("pk"))
    .order_by("-date").values("pk")[:1]
))
for bt, pt in qs.values_list("title", "post__title"):
    print(f"The latest post in {bt} is about {pt}")

However, how can I have Django construct Blog and Post objects out of such a queryset without needing additional queries? Something like the following:

for blog in qs.select_related("post"):
    print(f"The latest post in {blog.title} is about {blog.post.title}")

Assume it’s not possible to reverse the queryset (Post.objects.select_related("blog")), because, for example, there are other related models that need the same treatment as Post.

Answer 1

Surprisingly, this works!

qs = Blog.objects.annotate(
    latest_post=FilteredRelation(
        "post",
        condition=Q(
            post=Subquery(
                Post.objects.filter(blog=OuterRef("pk"))
                .order_by("-seq")
                .values("pk")[:1]
            ),
        ),
    ),
).select_related("latest_post")

for blog in qs:
    print(f"The latest post in {blog.title} is about {blog.latest_post.title}")

This doesn’t actually require the condition to be unique:

for blog in Blog.objects.annotate(
    some_post=FilteredRelation("post"),
).select_related("some_post"):
    do_something(blog, blog.some_post)
    # will happen multiple times for the same blog
    # but blog.some_post will be different each time