Ruby on Rails nil can't be coerced into BigDecimal

Question

Why do I get nil can't be coerced into BigDecimal when I try to perform a calculation: here's the code:

model/drink.rb

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

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

model/menu.rb

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

end

*Drink is the (nested)child model and Menu the parent model When I attempt to create a new drink the browser display following error message 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'

app/db/migration

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

controllers/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 

Please can anyone tell me what's wrong with the code?

Answer 1

If you want nil to be evaluated as 0.0 then you can do something like this:

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

Or explicitly check for 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 

The problem is really that your record fields aren't set like you expect them to be. Do you need to use validations to make sure that the price and quantity fields are set?

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

That way you ensure that you don't get a nil value when calling #total_amount.

Answer 2

You're not setting the quantity value anywhere so it is nil and nil cannot be coerced into a number for the multiplication.

Presumably your price is a decimal(n,2) (for some n) in the database so self.price is represented in Ruby as a BigDecimal object; that's why you're getting complaints about not being able to coerce to BigDecimal.

You can get a similar error out of irb like this:

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

Your self.price is set though. If it wasn't then you'd get one of these:

NoMethodError: undefined method `*' for nil:NilClass

You might want to add

validates_presence_of :quantity, :price

(or possible stricter validations) to your model if you're requiring that they be set.