The Elo handling from PorkChop, without the other stuff in the model. (Original source: https://github.com/PorkChopClub/porkchop/blob/master/app/models/player.rb)

player.rb

class Player < ActiveRecord::Base
  BASE_ELO = 1000
  
  has_many :elo_ratings
  after_save :record_rating
  attr_writer :elo

  def elo
    elo_ratings.most_recent_rating || BASE_ELO
  end

  def elo_on(date)
    elo_ratings.rating_on(date) || BASE_ELO
  end

  private

  def record_rating
    return unless @elo
    elo_ratings.create(rating: @elo)
  end
end