Order.rb

class Order
  # user
  # status %i{ registration payment waiting_products waiting_preorder packing sending complete }
  # wait_until_preorder bool - ждать для открыток?
  
  def avaiable?
    order_items.all? { |oi| oi.avaiable? }
  end
end

OrderController.rb

class OrderController
  def create
    order = Order.new(
      user: current_user,
      status: :registration,
      wait_until_preorder: params[:wait],
    )

    params[:cart].each do |line|
      product = Product.find line[:product_id]
      count = line[:count]
      order.order_items << OrderItem.new(
        pruduct: product,
        count: count,
      )

      order.weight += product.weight * count
      order.price += product.price * count
    end

    order.create!
  end

  def registrate
    order = Order.find(params[:order_id])
    order.address = params[:addres] # псевдо
    order.weight += 100 # за упаковку
    order.price += 100 # за упаковку
    order.status = :payment
    order.save!
  end

  def pay
    order = Order.find(params[:order_id])
    order.status = :waiting_products

    if order.avaiable?
      order.status = :waiting_preorder
    end

    if order.wait_until_preorder?
      unless order.order_items.any? { |oi| oi.preorder? }
        order.status = :packing
      end
    else
      order.status = :packing
    end
  end
end

OrderItem.rb

class OrderItem
  # order_id
  # product_id
  # count
  # reserved - сколько было зарезервировано для заказа
  # preorder_id

  def avaiable?
    reserved == count
  end

  def before_create
    if product.preorder?
      preorder = product.preorder
    end

    reserved = min(product.store, count)
    product.store -= reserved
  end
end

Product.rb

class Product
  def before_change_store
    product = Product.find(params[:product])
    OrderItems
      .includes(:order)
      .where(order: { status: :waiting_products })
      .where(product: product)
      .each do |oi|
        need = oi.count - oi.reserved
        oi.reserved += min(product.store, need)
        product.store -= oi.reserved
        oi.save!
        
        if oi.order.avaiable? { |oi| oi.avaiable? }
          oi.order.status = :waiting_preorder
        end

        if oi.rder.wait_until_preorder?
          unless oi.order.order_items.any? { |oi| oi.preorder? }
            oi.order.status = :packing
          end
        else
          oi.order.status = :packing
        end
        
        oi.order.save!
      end
  end
end