Set value in controller using nested resource

I have two models, product and order.

Product
 - cost
 - id

Order
 - cost
 - product_id

Each time someone places an order, it captures the product_id through a radio button value in the "new order" form.

In the controller when creating the new order it needs to set order.cost to order.product.cost. Logically I thought the code should be something like this:

def create
...
   @order.cost == @order.produc开发者_运维知识库t.cost
...
end

However I can't seem to make it work at all, hence me asking the question here.

Any help is answering (or naming) the question would be greatly appreciated.

Wrong syntax

@order.cost == @order.product.cost #it will compare the product cost & order cost & return boolean value true ot false

Its should be

@order.cost = @order.product.cost

Assuming you write associations properly in model it should be as follows

product.rb

has_many :orders

order.rb

belongs_to :product

Another option would be specifying a before_create on the Order model, but this would only work if EVERY Order needs to be created in this way.

class Order < ActiveRecord::Base
  has_many :products 
    #this could be has_one if you really want only one product per order
  accepts_nested_attributes_for :products
    #so that you can do Order.new(params[:order])
    #where params[:order] => [{:attributes_for_product => {:id => ...}}] 
    #which is handled by fields_for in the view layer.

    #has_one would make this :product

  before_create :calculate_order_cost_from_product 
    #only run on the first #save call or on #create

  def calculate_order_cost_from_product
      self.cost = self.products.first.cost 
       #this could also be self.products.sum(&:cost) 
       #if you wanted the total cost of all the products

       #has_one would be as you had it before:
       #self.cost = self.product.cost
  end

end