add_expiry_rule_migration.rb

ng_products = Product.where(:"commodity.name" => /ng/i).each do |p|
  p.expiry_rule = :preceeding_weekday
end

expiry.rb

module Expiry
  def preceeding_weekday(trade)
    [-5..-1].map{ |i| trade.tenor_start + i.days }.select(&:nerc_weekday?).last
  end
  
  # other expiry rules
end

product.rb

class Product
  include Expiry
  
  def expiry
    self.send(self.expiry_rule, self)
  end
end

trade.rb

class Trade
  delegate :expiry, to: :product
end