stats_preliminary_dsl.rb

class Order
  include Stats::Host

  # The above would mix this in:

  has_one :stat, class_name: "Order::Stat"
  after_commit :create_stat, on: :create

  def create_stat
    stat.create!
  end

end

class Earning
  include Stats::Provider

  stats_for :order,
    observe: [:amount, :earner_type]

  # The above would mix this in:

  after_commit :refresh_stat, on: :create
  after_commit :refresh_stat, on: :update, if: :should_refresh_stat?
  after_commit :refresh_stat, on: :destroy

  def refresh_stat
    order.stat.refresh
  end

  def should_refresh_stat?
    amount_changed? || earner_type_changed?
  end

end