preloading the latest of an associated collection

latest_association_by_distinct_on.rb

class Post
  has_many :comments
  has_one :latest_comment, -> { latest_by :post_id }, class_name: 'Comment'

class Comment
  scope :latest_by, -> column {
    from(
      order(column, created_at: :desc).arel
        .distinct_on(arel_table[column])
        .as(quoted_table_name))
  }

Post.preload(:latest_comment).each do |post|
  puts post.latest_comment.id
end