The WOW

Pure SQL.rb

# from https://stackoverflow.com/a/123481/3319298
# Uses LEFT JOIN to join the posts table with itself in a clever manner - on id AND the relevant, :created_at row.
# The check on created_at makes it so that the last records have NULLs in t2 rows, so we use WHERE to select those and done.
scope :users_last_posts, -> {

  query = <<~HEREDOC
    SELECT t1.id
    FROM post t1
    LEFT OUTER JOIN posts t2
      ON t1.author_id = t2.author_id AND
      (
        (t1.published_on < t2.published_on) OR
        (t1.published_on = t2.published_on AND t1.id < t2.id)
      )
    WHERE (
      t2.author_id IS NULL
    )
  HEREDOC

  query = query.gsub(%r'\n', ' ').gsub(%r'\s{2,}', ' ').strip

  joins("JOIN (#{query}) AS lasts ON lasts.id = #{table_name}.id")
}

Post.users_last_posts.order(created_at: :desc)