Question

I’m going through the Agile Web Development with Rails tutorial. There are Products, LineItems, and Carts.

Product

class Product < ActiveRecord::Base
  attr_accessible :description, :image_url, :price, :title
  has_many :line_items
end

LineItem

class LineItem < ActiveRecord::Base
  attr_accessible :cart_id, :product_id, :product
  belongs_to :product
  belongs_to :cart
end

Cart

class Cart < ActiveRecord::Base
  has_many :line_items, dependent: :destroy
end

LineItemsController

class LineItemsController < ApplicationController

  def create
    @cart = current_cart
    product = Product.find(params[:product_id])
    @line_item = @cart.line_items.build(product: product)
    ....
  end

my question is about the 3rd line in the create action above. I understand passing a product_id into line_items.build() but I don’t understand what passing an entire product does?

thanks,
mike

Answer 1

That will set the relation to the product (as in belongs_to product)

You could also just set the product_id by hand, but why would you. It is much simpler that way.

Active record understands the concept of relations – so it will do the setting of ids for you automatically.