Finding latest tweets by unique users

tweet.rb

##
# Find latest *n* tweets, but don't repeat tweets by users.
# Example:
#
# If we have the following table:
#
# id  |   user_id   |   created_at
# 1   |     1       |     3 days ago
# 2   |     2       |     3 days ago
# 3   |     1       |     2 days ago
# 4   |     1       |     2 days ago
# 5   |     3       |     1 day ago
# 6   |     2       |     1 day ago
#
# Then this would return something like this:
#
# id  |   user_id   |   created_at
# 6   |     2       |     1 day ago
# 5   |     3       |     1 day ago
# 4   |     1       |     2 days ago
#
#
# Limitations: This assumes that the greater id, the greater the date on the
# created_at field
class Tweet < ActiveRecord::Base
  scope :recent, lambda {|limit|
    where(id: select("max(tweets.id)").group("tweets.user_id").arel)
    .order("tweets.created_at DESC")
    .limit(limit)
  }
end