order_applier.rb

class ClaimApplier
  def initialize(order, claims)
    @order = order
    @claims = claims
  end

  def apply
    @claims.sort_by(&:expires_at).each do |claim|
      apply_claim(claim)
    end

    save_changed_claims
  end

  private

  def amount_to_apply(claim)
    [claim.value_remaining, @order.balance].min
  end

  def apply_claim(claim)
    amount = amount_to_apply(claim)

    @order.balance -= amount
    @order.claim_credit += amount
    claim.value_remaining -= amount
    create_redemption(claim, amount)
  end

  def create_redemption(claim, amount)
    if amount != 0
      redemption = Redemption.new
      redemption.claim = claim
      redemption.order = @order
      redemption.campaign = claim.campaign
      redemption.value = amount
      redemption.save!
    end
  end

  def save_changed_claims
    @claims.select(&:changed?).map(&:save)
  end
end