Question

Post has user and date attributes.

How can I turn this

posts = Post.objects.order_by('-date')[:30]

to give me 30 or less posts consisting of the last post by every user?

For example if I have 4 posts stored, 3 are from post.user=”Benny” and 1 from post.user=”Catherine” it should return 1 post from Benny and 1 from Catherine ordered by date.

Answer 1

I would probably use annotate, you might be able to use extra to get down to a single query.

posts = []
for u in User.objects.annotate(last_post=Max('post__date')).order_by('-last_post')[:30]:
    posts.append(u.post_set.latest('date'))

You could also use raw, which would let you write a SQL query, but still return model instances. For instance:

sql = """
SELECT * FROM app_post
WHERE app_post.date IN
    (SELECT MAX(app_post.date) FROM app_post
     GROUP BY app_post.user_id)
ORDER BY app_post.date DESC
"""
posts = list(Post.objects.raw(sql))