@H_403_1@当我尝试执行计算时,为什么无法将nil强制转换为BigDecimal：这是代码：

模型/ drink.rb

class Drink < ActiveRecord::Base
  belongs_to :menu 
  before_save :total_amount 

def total_amount
    self.total_amount = self.price * self.quantity
end

模型/ menu.rb

class Menu < ActiveRecord::Base
    has_many :drinks,:dependent => :destroy
    accepts_nested_attributes_for :drinks,:allow_destroy => true
    #Validations

end

* Drink是(嵌套)子模型,Menu是父模型当我尝试创建新饮料时,浏览器显示以下错误消息nil无法强制进入BigDecimal app / models / drink.rb：7：in’total-量’
app / controllers / menus_controller.rb：47：’create’
app / controllers / menus_controller.rb：46：’create’

应用程序/分贝/迁移

class CreateDrinks < ActiveRecord::Migration
  def change
    create_table :drinks do |t|
      t.string :name
      t.decimal :quantity,:precision => 8,:scale => 2
      t.decimal :price,:scale => 2
      t.decimal :vat,:scale => 2
      t.references :menu

      t.timestamps
    end
    add_index :drinks,:menu_id
  end
end

控制器/ drinks_controller.rb

class DrinksController < ApplicationController
      # GET /drinks
      # GET /drinks.json
      def index
        @drinks = Drink.all

        respond_to do |format|
          format.html # index.html.erb
          format.json { render :json => @drinks }
        end
      end

      # GET /drinks/1
      # GET /drinks/1.json
      def show
        @drink = Drink.find(params[:id])

        respond_to do |format|
          format.html # show.html.erb
          format.json { render :json => @drink }
        end
      end

      # GET /drinks/new
      # GET /drinks/new.json
      def new
        @drink = Drink.new

        respond_to do |format|
          format.html # new.html.erb
          format.json { render :json => @drink }
        end
      end

      # GET /drinks/1/edit
      def edit
        @drink = Drink.find(params[:id])
      end

      # POST /drinks
      # POST /drinks.json
      def create
        @article = Drink.new(params[:drink])

        respond_to do |format|
          if @drink.save
            format.html { redirect_to @drink,:notice => 'Drink was successfully created.' }
            format.json { render :json => @drink,:status => :created,:location => @article }
          else
            format.html { render :action => "new" }
            format.json { render :json => @drink.errors,:status => :unprocessable_entity }
          end
        end
      end

      # PUT /drinks/1
      # PUT /drinks/1.json
      def update
        @drink = Drink.find(params[:id])

        respond_to do |format|
          if @drink.update_attributes(params[:drink])
            format.html { redirect_to @drink,:notice => 'Drink was successfully updated.' }
            format.json { head :ok }
          else
            format.html { render :action => "edit" }
            format.json { render :json => @drink.errors,:status => :unprocessable_entity }
          end
        end
      end

      # DELETE /drinks/1
      # DELETE /drinks/1.json
      def destroy
        @drink = Drink.find(params[:id])
        @drink.destroy

        respond_to do |format|
          format.html { redirect_to drinks_url }
          format.json { head :ok }
        end
      end 
    end

请问有谁可以告诉我代码有什么问题？

def total_amount
    self.total_amount = self.price.to_s.to_d * self.quantity.to_s.to_d
end

def total_amount
  if self.price && self.quantity
    self.total_amount = self.price * self.quantity
  else
    self.total_amount = "0.0".to_d
  end
end

class Drink
  validates :price,:presence => true      # Don't forget add DB validations,too :)
  validates :quantity,:presence => true
end