Grab only the latest comment in Rails - ruby-on-rails

Question

In a typical User - Post - Comment model in Rails, every user can create a Post and also can create Comment, question is how to grab every user latest comment on specific post.
Example:
Post A have 3 user making comment
User 1 have comment 1, 2, 3, 4, 5, 6
User 2 have comment 1, 2, 3, 4
User 3 have comment 1, 2
So the view I want is just the latest comment for every user:
Post A have 3 user making comment
User 1 latest comment that is 6
User 2 latest comment that is 4
user 3 latest comment that is 2
How to do it ?
thanks

Answer 1

Something like this:
post.comments.for_user(current_user).last
add a named_scope in your model
class Comment
named_scope :for_user, lambda{ |user| {:conditions=>{:user_id => user.id}}
end
That should do the trick.
If you rather do it in rails,
messages_by_users = post.messages.group_by(&:user)
messages_by_users.each do |key, value|
messages_by_users[key] = value.last
end

Answer 2

I have had to get this kind of data and usually I end up doing two queries. In my case I have Blogs and their Posts and I wanted a list of the 3 most recent blog posts with the restriction that the blogs are unique, I dont want 2 posts from the same blog. I ended up doing something like this (MySQL):
q = <<-EOQ
SELECT id,pub_date FROM
(
SELECT id,blog_id,pub_date
FROM posts
ORDER BY pub_date DESC
LIMIT 40
)
t
GROUP BY blog_id
ORDER BY pub_date DESC
LIMIT #{num_posts}
EOQ
post_ids = Post.connection.select_values(q)
Post.find(:all, :include => [:blog], :conditions => ["id IN (?)", post_ids], :order => "posts.pub_date DESC")
So in your case you might have something like:
q = <<-EOQ
SELECT id FROM
(
SELECT id,post_id
FROM comments
ORDER BY id DESC
LIMIT 40
)
t
GROUP BY post_id
ORDER BY id DESC
LIMIT 10
EOQ
post_ids = Post.connection.select_values(q)
Post.find(:all, :include => [:blog], :conditions => ["id IN (?)", post_ids], :order => "posts.id DESC")

Answer 3

Assuming that your database is assigning sequential IDs to the comments, you can do this:
class Comment
named_scope :most_recent, lambda {
lastest_comments = Comment.maximum :id, :group => "user_id, post_id"
{ :conditions => [ "comment_id in ?", lastest_comments.map(&:last) ] }
}
end
This gives you a two-query method that you can use in a variety of ways. The named_scope above pulls back the most recent comments for all users on all posts. This might be a problem if your database is gigantic, but you can certainly add conditions to make it more specific.
As it stands, it is a flexible method that allows you to do the following:
Comment.most_recent.find_by_user #user #-> the most recent comments on all posts by a user
#user.comments.most_recent #-> same as above
Comment.most_recent.find_by_post #post #-> the most recent comments on a single post by all users
#post.comments.most_recent #-> same as above
Comment.most_recent.find_by_user_and_post #user, #post #-> the specific most recent comment by a certain user on a certain post
#post.comments.most_recent.find_by_user #user #-> you get the idea