How can I pull latest posts, with unique authors, so that if an author wrote two posts in a row, it only gets the first?

Question

So right now, I have Posts and Authors... I want to make a "20 recent posts" section at my website.

Sometimes an author makes 4-5 posts in a row. In the 20 latest posts, I want all of these posts to have unique authors. So if author A wrote last 3 posts based on created_at, I only want the last post.

Hypothetically:

A wrote posts 10, 9, 7 B wrote posts 8, 6 C wrote posts 5, 1 D wrote posts 4, 3, 2

Then in the latest 4 posts section, I'd like posts: 10, 8, 7, 6

Or in any other manner where the author ID is unique, and I have 20 latest posts.

I've tried Post.select(:author_id).distinct, mapping, and other bits here and there, but I'm not sure if I'm going the right direction.

Thanks guys.

Answer 1

This is the general idea. You can fill in the details.

 select author, maxts, etc
 from sometables join
 (select authorId, max(timestamp_field) maxts
 from etc
 where whatever
 group by authorId) temp on temp.authorId = author.authorId
 and sometable.timestamp_field = maxts
 where whatever

Make sure the "where whatever" is the same in both places.