Ruby on Rails calculation

Question

Actually I'm trying to perform a multiplication on a project.I have 2 models : a parent model and a child model.I want to show the result of the multiplication performed on the child model in the parent view by invoking the multiplication method.Here's the code:

app/models/drink.rb aka child model

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

  def total_amount
    self.quantity * self.price * 1.30
end   
end

> in app/models/menu.rb aka parent model

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

end

in views/menu/show.html.erb

<td><%=number_to_currency(@menu.total_amount) %> </td>

and the error message:

undefined method `total_amount' for nil:NilClass

Obviously total_amount is an attribute of the model drink .What am I doing wrong.Thanks for helping.

Have you tried @menu.drinks.total_amount ?

Misha M
– Misha M

2011-10-14 23:37:51 +00:00
Commented Oct 14, 2011 at 23:37 — Misha M
– Misha M, Commented Oct 14, 2011 at 23:37

sosborn · Accepted Answer · 2011-10-15 04:01:52Z

2

Obviously total_amount is an attribute of the model drink .

Yeah, that method is in the drink model (by the way, if it is an attribute then it is declared in your migration. The way you have it set up the total_amount will not be saved in the database), but, you are calling it on an instance of Menu.

As Misha M said, @menu.drinks.total_amount.

edited Oct 15, 2011 at 4:01

answered Oct 15, 2011 at 0:04

sosborn

14.7k2 gold badges44 silver badges46 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

blawzoo Over a year ago

When I put @menu.drinks.total_amount, I get a message error: undefined method 'total_amount' for <ActiveRecord::Relation:0x10318aae8>, please can you tell me what's wrong.Is it a migration issue?

sosborn Over a year ago

Ok, I didn't get what you were trying to do until I read your response to Mitch. Ok, if you want the total_amount for a specific drink then you need a call that method on a specific drink. @menu.drinks returns all of the drink on that menu. Instead, you want something like @menu.drinks[0], which will return the first drink in the @menu.drinks relation.

blawzoo Over a year ago

@menu.drinks[0] displays #<Drink:0x1035f51f0> on view. which is the id of instance of drink (I guess?).That's great, but more than that I want each instance of drink perform total_amount(which is a method of model/drink.rb and an attribute of it). So How can I simply perform a child model calculation method in a parent model view(the method is an attribute of the child model too)?

sosborn Over a year ago

As I said, call the method on the instance. @menu.drinks[0].total_amount. If you want more than one drink to do this then you have to loop through all the drinks and call the method on each one.

Mitch Dempsey · Accepted Answer · 2011-10-14 23:52:49Z

1

You need to add a function called total_amount to your Menu class, and then have it iterate through all the drinks, and sum the total amount of each drink.

answered Oct 14, 2011 at 23:52

Mitch Dempsey

40.3k6 gold badges72 silver badges75 bronze badges

1 Comment

blawzoo Over a year ago

matter fact I don't want to sum up the total_amount of each drink.total_amount is the total of one single drink price not the sum of all drink

Community · Accepted Answer · 2017-05-23 12:15:54Z

Matter fact the problem is that the quantity and the price of the drink was not set.A part of the solution comes from Wizard of Ogz.Matter fact by trying to solve a "nil can't be coerced into BigDecimal" I get the solution to this issue too.So here's is the solution

1-app/models/drink.rb aka child model Apply to self.quantity and self.price a method which convert them to string(to_s) then to big decimal(to_d)

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

  def total_amount
    self.quantity.to_s.to_d * self.price.to_s.to_d * 1.30
  end   
end

2-app/models/drink.rb aka child model validates the presence of price and quantity before saving them to the database

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

      validates :price, :presence => true 
      validates :quantity, :presence => true 

      def total_amount
        self.quantity.to_s.to_d * self.price.to_s.to_d * 1.30
      end   
end

3-app/views/menus/show.html.erb aka parent model

Simply apply the method total_amount to dink aka child(nested) model as following:

<td><%=number_to_currency(drink.total_amount) %> </td>

Thanks to Wizard of Ogz, Mitch Dempsey, sosborn, and Misha M

Collectives™ on Stack Overflow

Ruby on Rails calculation

3 Answers 3

4 Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related