rails way to calculate subtotal and total

Question

Context: I have 2 models Order and Item.

I want to calculate Item subtotal based on item.quantity * item.price For now this is done in the view (but not the appropriate place).

<%= number_to_currency(item.quantity * item.price) %>

I also need to calculate the Order total but I'm stuck. I don't have any column for that. What's the best approach? Use the model? Helper or Observer?

For now I managed to have subtotal working through Order helper

def item_subtotal(item)
  item_subtotal = item.quantity * item.price
end

Working Solution:

Item model

def subtotal
  price * quantity
end

In View render <%= item.subtotal %>

Order model

def total_price
  total_price = items.inject(0) { |sum, p| sum + p.subtotal }
end

Order#show view render <%= number_to_currency(@order.total_price) %>

Answer 1

Since it is a functionality of the model (you want to calculate something of the item that is self-referential), the more appropriate location is the model itself, so that you can easily use

item.subtotal

Answer 2

On your Item model you could add a subtotal method:

def subtotal
  quantity * price
end

Assuming that you're Order model as a has_many relationship with Item you could map the collection to get a list of prices on the Order model:

def subtotals
  items.map do |i| i.subtotal end
end

Because you're running in a Rails environment you can use the activesupport sum method to get the total on an Order model:

def total
  subtotals.sum
end

Or if you prefer putting it all together:

def total
  items.map do |i| i.subtotal end.sum
end

Then you can use the subtotal on Item and total on Order in your views.

Edit: The view could look like this:

<% for item in @order.items %> <%= item.price %><br/><% end %>
<%= @order.total %>

Answer 3

You can Show the item_subtotal by using

and for grand total you can calculate it by

total_price = @order.items.to_a.sum { |item| total = item.product.price*item.quantity }

I think this will work for you.