Rails模型中项目的总和-无类错误

我正在尝试将产品模型和服务模型中的值添加到订单模型中。我将在下面发布模型,这样关系就清晰了。我尝试过多种选择,但本质上,如果我添加了一个产品项目而没有服务项目,它就会崩溃,反之亦然。这里的信息: NoMethodError (undefined method `price' for nil:NilClass):

to_i here

class Order < ApplicationRecord
  has_many :line_items
  before_save :update_total
  before_update :update_total
  belongs_to :user, optional: true

  def total_service_items
    self.line_items.collect { |item| item.service.price.to_i * item.quantity }.sum
  end

  def total_product_items
    self.line_items.collect { |item| item.product.price.to_i * item.quantity }.sum
  end

  def calculate_total
    total_service_items + total_product_items
  end

  def update_total
    self.total_price = calculate_total
  end    
end

class LineItem < ApplicationRecord
  belongs_to :order, optional: true
  belongs_to :product, optional: true
  belongs_to :service, optional: true
end

class Service < ApplicationRecord
    has_many :line_items
end

class Product < ApplicationRecord
    has_many :line_items
end

Create

class LineItemsController < ApplicationController     

  def create
    @order = current_order
    @item = @order.line_items.new(item_params)
    @order.save
  end

      def item_params
        params.require(:line_item).permit(:quantity, :service_id, :product_id, :unit_price, :order_id)
    end
end