Question

I have a Post model that has_many :comments. The question is: how can I build a query using ActiveRecord Query Interface that retrieves all posts and the last comment of each posts?

All I have right now is the following, but it doesn’t filter the comments and I have no idea of what to do next:

Post.includes(:comments)

Answer 1

Well, the simplest thing comes to mind, which may not be the best is creating a scope.

class Post < ActiveRecord::Base
    # has_many or maybe a has_one ? I don't have time to fire up console again to check, sorry!
    has_many :last_comment, :class_name => 'Comment', :limit => 1, :order => 'comments.created_at DESC'
end

And then use

Post.includes(:last_comment)

Be sure you have indexes in your SQL.