Question

How it is better to count for each line item it’s total cost, and then for all count total summary? I have such db: article has table existence where i have prices, but how to display price * quantity? it’s better to do in controller? or in model, but when in model, how to do this, note, in db I have only price, and quantity is given in line item:
I want something like this:

class LineItem < ActiveRecord::Base
    belongs_to :article
    belongs_to :cart
    def total_price
      existence.PRICEM * quantity
    end
end

Answer 1

This core business functionality should reside in your models.

Your cart could have:

class Cart
  def total_price
    line_items.inject(0.0){|sum,line_item| sum + line_item.total_price }
  end
end

class LineItem
  def total_price
    quantity * price
  end
end

This way you can still loop over your line-items to show their price, and then your cart as well. As this transfers to an order, you’ll want to store those values rather than calculate them.