# Sum the Product of Two Fields **URL:** \ **Category:** rubyonrails-talk\ **Created:** [February 19, 2008, 3:34am UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111 "2008-02-19T03:34:55Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![11175](https://avatars.discourse-cdn.com/v4/letter/1/8edcca/32.png) [@11175](https://discuss.rubyonrails.org/u/11175)\ **Post date:** [February 19, 2008, 3:34am UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111/1 "2008-02-19T03:34:55Z")
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. ---
**Author:** ![Steve\_Ross](https://avatars.discourse-cdn.com/v4/letter/s/3d9bf3/32.png) [@Steve\_Ross](https://discuss.rubyonrails.org/u/Steve_Ross)\ **Post date:** [February 19, 2008, 3:58am UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111/2 "2008-02-19T03:58:21Z")
You might prefer to use inject. self.line\_items.inject{|a, li| a += li.quantity \* li.price} ---
**Author:** ![11175](https://avatars.discourse-cdn.com/v4/letter/1/8edcca/32.png) [@11175](https://discuss.rubyonrails.org/u/11175)\ **Post date:** [February 19, 2008, 4:01am UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111/3 "2008-02-19T04:01:09Z")
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. ---
**Author:** ![AndyV1](https://avatars.discourse-cdn.com/v4/letter/a/5fc32e/32.png) [@AndyV1](https://discuss.rubyonrails.org/u/AndyV1)\ **Post date:** [February 19, 2008, 3:47pm UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111/4 "2008-02-19T15:47:41Z")
@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. ---
**Author:** ![11175](https://avatars.discourse-cdn.com/v4/letter/1/8edcca/32.png) [@11175](https://discuss.rubyonrails.org/u/11175)\ **Post date:** [February 20, 2008, 2:22am UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111/5 "2008-02-20T02:22:32Z")
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 #\ 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. ---
**Author:** ![Steve\_Ross](https://avatars.discourse-cdn.com/v4/letter/s/3d9bf3/32.png) [@Steve\_Ross](https://discuss.rubyonrails.org/u/Steve_Ross)\ **Post date:** [February 20, 2008, 2:32am UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111/6 "2008-02-20T02:32:54Z")
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 ---
**Author:** ![AndyV1](https://avatars.discourse-cdn.com/v4/letter/a/5fc32e/32.png) [@AndyV1](https://discuss.rubyonrails.org/u/AndyV1)\ **Post date:** [February 20, 2008, 7:17pm UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111/7 "2008-02-20T19:17:09Z")
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. ---
**Author:** ![11175](https://avatars.discourse-cdn.com/v4/letter/1/8edcca/32.png) [@11175](https://discuss.rubyonrails.org/u/11175)\ **Post date:** [February 21, 2008, 12:58am UTC](https://discuss.rubyonrails.org/t/sum-the-product-of-two-fields/25111/8 "2008-02-21T00:58:07Z")
@Steve, @Andy - thanks for the help. You guys are great.