Question

为什么我在尝试执行计算时会得到nil can't be coerced into 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品时，浏览器显示以下错误消息nil can't be coerced into BigDecimal app/models/drink.rb:7:in 'total-amount' app/controllers/menus_controller.rb:47:in 'create' app/controllers/menus_controller.rb:46:in '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, :precision => 8, :scale => 2
      t.decimal :vat, :precision => 8, :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

任何人都可以告诉我代码有什么问题吗？

Answer 1

如果您希望将nil评估为0.0，那么您可以执行以下操作：

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

或明确检查nil

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

问题实际上是你的记录字段没有像你期望的那样设置。您是否需要使用验证来确保price和quantity字段已设置？

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

这样你就可以确保在调用#total_amount时没有获得nil值。

Answer 2

您未将quantity值设置为任何位置，因此nil和nil无法强制转换为乘法数字。

据推测，您的price在数据库中是decimal(n,2)（对于某些n），因此self.price在Ruby中表示为BigDecimal对象;这就是为什么你得到关于不能强迫BigDecimal的投诉。

你可以从irb中得到类似的错误：

>> 11 * nil
TypeError: nil can't be coerced into Fixnum
    from (irb):7:in `*'

虽然您设置了self.price。如果不是那么你会得到其中一个：

NoMethodError: undefined method `*' for nil:NilClass

您可能想要添加

validates_presence_of :quantity, :price

（或可能更严格的验证）如果您要求设置它们，您的模型。

Ruby on Rails nil无法强制转换为BigDecimal

2 个答案: