[Fixed]-Django annotating with a first element of a related queryset

You can use a Subquery to annotate on a single field from the most recent related object:

comments = Comment.objects.filter(
    post=OuterRef('pk')
).order_by('-timestamp').values('timestamp')
Post.objects.annotate(
    last_comment_time=Subquery(comments[:1])
)

You could annotate on several fields this way, but that would hurt performance (each correlated subquery runs separately, and for each row, which is better than N+1 queries, but worse than a single join).

You can build up a JSON object on a single field, and then annotate that on:

comments = Comment.objects.filter(
    post=OuterRef('pk')
).annotate(
    data=models.expressions.Func(
        models.Value('author'), models.F('author'),
        models.Value('timestamp'), models.F('timestamp'),
        function='jsonb_build_object',
        output_field=JSONField()
    ),
).order_by('-timestamp').values('data')

(It’s even possible to get the whole object as JSON, and then re-inflate that in Django, but that’s a bit hacky).

Another solution could be to fetch the most recent comments seperately, and then combine them with the posts:

comments = Comment.objects.filter(
    ...
).distinct('post').order_by('post', '-timestamp')
posts = Post.objects.filter(...).order_by('pk')

for post, comment in zip(posts, comments):
    pass

You would need to make sure the posts and comments are in the same order here: these queries are. This would also fail if there was not a comment on each post.

A workaround for that could be to put the comments into a dict keyed by post id, and then fetch the matching one for each post.

comments = {
    comment.post_id: comment
    for comment in Comment.objects.distinct('post').order_by('post', '-timestamp')
}
for post in Post.objects.filter(...):
    top_comment = comments.get(post.pk)
    # whatever