Calculate total price (items quantity * items price)-CodePudding

I have a Campaign model that has_many items.

Each item has :quantity and :price

I want to calculate the total price for the Campaign.

I tried with this but won't work.

  def campaign_subtotal(campaign)
    return campaign.items.sum(item.price * item.quantity)
  end

How is this approached?

Thanks a lot

CodePudding user response：

The naive implementation is:

class Campaign < ApplicationRecord
  has_many :items
  def subtotal
    items.sum { |item| item.price * item.quantity }
  end
end

You were somewhat close but you need to pass a block to calculate the sum per item. It also makes more sense to make this an instance method rather then a helper.

However if you want to use the subtotal in database queries or you want to display a bunch of campaigns efficiently you would want to calculate it in SQL with a subquery, window query or lateral join. You should also consider recording the price per line (the subtotal) instead of recalculating it all the time.