Sum the Product of Two Fields

rubyonrails-talk
1

I have two MySQL tables, Orders and Line_items. Orders has many line_items. The line_item table has a quantity field and an item_price field. In order.rb, I want to get the total price of the order. I try

  def total_price     self.line_items.sum { |li| li.quantity * li.item_price}   end

and get "Wrong number of argument (1 for 2)

  def total_price     self.line_items.each { |li| tpsum += (li.quantity * li.item_price)}     tpsum   end

"you have a nil item when you didn't expect it. you might have expected an array..."

  def total_price     self.line_items.each do |li|       tpsum = tpsum + (li.quantity * li.item_price)     end     tpsum   end

same error - nil when you didn't expect it.

If I do

self.line_items.sum(:quantity)

I get an answer. That makes me think that I can treat line_items like an array. How can I sum the product of two fields? Thanks.

2

You might prefer to use inject.

self.line_items.inject{|a, li| a += li.quantity * li.price}

3

Hello,

have you tried it?

   def total_price      @all_items = LineItems.find :all      @all_items.inject(0) { |sum, li| sum + (li.quantity * li.item_price}    end

I not check my script above, i hope you can inform me if found any error.

4

@Dick,

In your first attempt I believe the problem is that you're working against the association proxy. It looks a lot like an array, but it's not. When you ask it to 'sum', it's actually trying to trigger a sum in the DB, rather than iterating through the line items.

In the subsequent attempts, the problem is one of scope. tpsum is local to the block that you're using. When you attempt to return it, however, you're outside the block and in a context that knows nothing about tpsum. You could fix any of those solutions by simply initializing tpsum=0 before the block.

s.ross, though, posts the more elegant solution.

5

def total_price     #self.line_items.inject{|a, li| a += li.quantity * li.item_price}      @all_items = self.line_items.find :all      @all_items.inject(0) { |sum, li| sum + (li.quantity * li.item_price)}   end

@Steve - I get the following error when I use the one-liner that's commented out above.

undefined method `+' for #<LineItem:0x4654ccc>

When I change it to

self.line_items.inject(0) {|a, li| a += (li.quantity * li.item_price)}

It works fine. I plan to do some reading on inject (first time I've heard of it - kind of a noob), but if you've got any pointers I'd love the head start.

@Reinhart - works as implemented above, thanks.

@AndyV - thanks for the explanation. Local to the block, huh? I'm not used to that. I better skip ahead to the "introduction to ruby" section of my book. Thanks again.

6

I think you hit on it when you used 0 as the initializer for inject. By default, inject is supposed to used the first element of the enumerable as the starting value of the accumulator. That gives you a starting "a" of type LineItem, which is clearly not the intended type. By explicitly initializing it with a Fixnum, 0, you have created an accumulator (or memo) with a compliant type.

So the corrected code:

self.line_items.inject(0) {|a, li| a += li.quantity * li.item_price }

should work without the parentheses around the product.

The inject method is quite idiomatic and if you look at some of the inject/reject/select/map/zip methods, it will remind you of Python or perhaps of certain aspects of functional programming. If you're not into that, inject is not a really obvious choice.

--steve

7

Technically, it's local to the scope in which it was created, and the block has it's own scope that's 'nested' within the scope of the total_price method. So... you could declare it in total_price and the block would see it, but not the other way around.

8

@Steve, @Andy - thanks for the help. You guys are great.