Question

I have some data inside this variable and I want to know how could I write an method to give me the sum of the items price.

Model names and their associations:

Order
- belongs_to :customer
- has_many :item_orders
- has_many :items, :through => :item_orders
- accepts_nested_attributes_for :item_orders, :allow_destroy => true, :reject_if => lambda { |a| a['item_id'].blank? }


Item
- has_many :item_orders, :dependent => :destroy
- has_many :orders, :through => :item_orders
- accepts_nested_attributes_for :item_orders, :allow_destroy => true

ItemOrder
- belongs_to :item
- belongs_to :order

doing:

<%= debug @order.items %> 

# returns me

- !ruby/object:Item
  attributes:
  id: 31
  title: Golden Ring 2
  price: 13445.0
  labour_cost: 500.0
  item_type_id: 10
  created_at: 2011-08-13 10:53:24.000000000Z
  updated_at: 2011-08-18 06:10:36.000000000Z
  photo: golden_ring.jpg
  changed_attributes: {}
  previously_changed: {}
  attributes_cache: {}
  marked_for_destruction: false
  destroyed: false
  readonly: false
  new_record: false
- !ruby/object:Item
 attributes:
  id: 32
  title: Special Pendant
  price: 171.67
  labour_cost: 120.0
  item_type_id: 20
  created_at: 2011-08-13 11:09:43.000000000Z
  updated_at: 2011-08-14 06:03:02.000000000Z
  photo: prd_194_48cd9b21771dd.jpg
  changed_attributes: {}
  previously_changed: {}
  attributes_cache: {}
  marked_for_destruction: false
  destroyed: false
  readonly: false
  new_record: false

So, I want to write a method in the Order model class to make possible writes something like this on the view.

 <%= @order.items.total_price %>

The total_price method will sum all the item prices and multiply by the quantity that contains in @order.items.

My doubt is: from inside the method, how could I have access to the items collection to perform the sum operation.

def total_price
  self.item_order.each do |i| { 
     result += i.quantity * i.items.price
  }
end

EDIT: I totally forgot to mention that I have to consider the quantity also to make this math.

Answer 1

You have two possible approaches.

1) Use ActiveRecord sum. This is efficient as the SUM operation is performed in the DB. When you are dealing with 100s of rows this is the way to go:

items.sum(:price)  # executes select sum(items.price) 
                   #          from items where items.order_id = some_id

2) Use Enumeration sum. Here you are operating on an existing array.

items.to_a.sum(&:price)

I use this approach when my has_many association list size is small( < 10) and already eager loaded.

Edit 1: Based on additional requirement

Using ActiveRecord:

item_orders.sum("item_orders.quantity * item.price", :join => :item)

Using Enumeration

item_orders.all(:include => :item).sum {|io| io.quantity * io.item.price }