Active Record LIMIT within GROUP_BY - sql

Question

SCENARIO
I have a table full of posts with a users table.
I want to be able to fetch all the posts and group them by users but I want to set a limit of say 10 per user.
class Post < ActiveRecord::Base
belongs_to :user
end
class User < ActiveRecord::Base
has_many :posts
end
# I thought this might work but it just grabs the first 10 posts and groups them
Post.find(:all, :limit=>10).group_by(&:user)
Any thoughts? Do I have to write custom SQL for or can Active Record do this?

Answer 1

Something like?
Post.group(:user_id).limit(10)

Answer 2

Post.group(:user_id).limit(10)
group_by is not a query method, but rather a method of Enumerable.
In your code, Post.find(:all, :limit => 10) is turned into an Array before being passed to group_by. The method above chains query methods together and only converts them to an Array when you need to use them.
ActiveRecord handles the whole thing. The above method translates to
SELECT `posts`.* FROM `posts` GROUP BY user_id LIMIT 10

Answer 3

The only way I know to grab the recent 10 posts per user would require a nested sub-query (which can have performance issues) or postgres-style lateral join. Fairly confident this cannot be accomplished with only active-record and requires writing custom SQL, which you've indicated you want to avoid.
As an alternative that could be accomplished without custom SQL, you could list each user and their posts within a time window (e.g. past month or year) with the following:
class User < ActiveRecord::Base
has_many :recent_posts, -> { where(posts: {created_at 1.month.ago..Time.now}) }, class_name: 'Post'
end
User.includes(:recent_posts).each do |user|
user.recent_posts
end
Which would not execute a SQL query for each user and would therefore be relatively performant compared with doing it purely in ruby.