Strange Sum issue

rubyonrails-talk
1

Hello,

This may just be my misunderstanding something, but I'm seeing something strange.

I have a model, Ticket, that in the database table 'tickets' has a field, let's say 'seats'.

I can do a Find and retrieve the items:

   t = Ticket.find :all

Then do a sum to get the total seats:   s = t.sum(&:seats)

and that returns the correct sum.

Playing in the console, I did:   t.sum(&:seats)

and while returning the correct sum, it also changed the 't' variable to be just the final item in the array. Is this correct? The sum is just doing an 'inject', which I didn't think altered the original array, but maybe I'm wrong?

And here's a second question, say I want to use the found items, but restrict the sum to a subset - total all seats that have been sold. I could do it by doing another find, but can I use .inject with a condition, so I don't have to requery the database?

Thanks!

2

Hmm, not sure on the console based sum findings, I don't see the same behavior. On your other question, the sum methods, etc all use ActiveRecord::Calculations#calculate and it will take a conditions parameter. See: http://api.rubyonrails.org/classes/ActiveRecord/Calculations/ClassMethods.html#M000969

If your condition is fairly simple and you can just use inject manually:

t = Ticket.find :all

s = t.inject{|sum,n| n.column_you_care_about == 'some_value' ? sum + n.column_to_add : 0}

-Bill

John Tsombakos wrote:

3

Thanks. I think the weird console behavior may have just been my stupidity :slight_smile: I think I was using the array inside the inject so it was just assigning to itself. Duh.

I'm trying the simple inject, but for some reason it isn't totaling the column. I'm using:

s= t.inject(0) {|total,n| n.picked_up ? (total + n.seats) : 0}

but it's always coming out 0. (seats is a boolean, and I'm (for now) using sqlite, so the actual data is 't' or 'f' - but looking at one record displays correctly - t[2].picked_up is true) I tried using n.picked_up? and n.picked_up == true

Doing: t[2].picked_up ? "Y" : "N" correctly returns "Y"

Thanks,

jt

4

Hello,

This may just be my misunderstanding something, but I'm seeing something strange.

I have a model, Ticket, that in the database table 'tickets' has a field, let's say 'seats'.

I can do a Find and retrieve the items:

   t = Ticket.find :all

Then do a sum to get the total seats:   s = t.sum(&:seats)

It is *much* more efficient to use the AR calculation methods:

  s = Ticket.sum(:seats)

The first one retrieves *all* of the rows and instantiates an AR object for each. The second issues a single query that returns one number.

and that returns the correct sum.

Playing in the console, I did:   t.sum(&:seats)

and while returning the correct sum, it also changed the 't' variable to be just the final item in the array. Is this correct? The sum is just doing an 'inject', which I didn't think altered the original array, but maybe I'm wrong?

Dunno. I don't see that behavior. The sum() method you're using there is not the ActiveRecord::Base.sum method, but the Enumerable#sum method added by active support.

5

> > Hello, > > This may just be my misunderstanding something, but I'm seeing > something strange. > > I have a model, Ticket, that in the database table 'tickets' has a > field, let's say 'seats'. > > I can do a Find and retrieve the items: > > t = Ticket.find :all > > Then do a sum to get the total seats: > s = t.sum(&:seats)

It is *much* more efficient to use the AR calculation methods:

  s = Ticket.sum(:seats)

The first one retrieves *all* of the rows and instantiates an AR object for each. The second issues a single query that returns one number.

True, but I had already performed the Find that would then be passed along to my view, so I figured one less trip to the database - and if I also want a sum of seats not picked up, I'd then make yet another trip to the DB.

> > and that returns the correct sum. > > Playing in the console, I did: > t.sum(&:seats) > > and while returning the correct sum, it also changed the 't' variable > to be just the final item in the array. Is this correct? The sum is > just doing an 'inject', which I didn't think altered the original > array, but maybe I'm wrong?

Dunno. I don't see that behavior. The sum() method you're using there is not the ActiveRecord::Base.sum method, but the Enumerable#sum method added by active support.

Yeah, I think that was just my dumb error.

I also got the .inject method to work by doing instead:

s= t.inject(0) {|total,n| total + (n.picked_up ? n.seats : 0)}

6

the result of your block needs to be the running total - not the value of the individual term, ie

n.picked_up ? (total + n.seats) : total

Fred

7

Ahhh.. thanks for the clarification.. I had a feeling I was missing something simple :slight_smile:

8

Sorry Tom, this was my fault. There way a typo in my original post. Glad you got it working.

-Bill

John Tsombakos wrote: