# Using .sum

**URL:** <https://www.ruby-forum.com/t/using-sum/180329>\
**Category:** Rails\
**Created:** [December 17, 2009, 4:00pm UTC](https://www.ruby-forum.com/t/using-sum/180329 "2009-12-17T16:00:51Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![mercurius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c89c15/32.png) [@mercurius](https://www.ruby-forum.com/u/mercurius)\
**Post date:** [December 17, 2009, 4:00pm UTC](https://www.ruby-forum.com/t/using-sum/180329/1 "2009-12-17T16:00:51Z")

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I have three tables: artists, albums, songs. What I want is to find the  
total duration of all songs in a given album.

> Artist.first.albums.first.songs.collect{|x|x.duration}

Of course, the above generates an array containing the durations of all  
songs in the first album of the first artist:

> [“4.05”, “2.36”, “3.31”, “3.25”, “2.45”, “4.58”]

What is the method I can use to total these?

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<div class="post-metadata">

**Author:** ![mercurius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c89c15/32.png) [@mercurius](https://www.ruby-forum.com/u/mercurius)\
**Post date:** [December 17, 2009, 4:54pm UTC](https://www.ruby-forum.com/t/using-sum/180329/2 "2009-12-17T16:54:49Z")

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Pale H. wrote:

> I have three tables: artists, albums, songs. What I want is to find the  
> total duration of all songs in a given album.
> 
> > Artist.first.albums.first.songs.collect{|x|x.duration}
> 
> Of course, the above generates an array containing the durations of all  
> songs in the first album of the first artist:
> 
> > [“4.05”, “2.36”, “3.31”, “3.25”, “2.45”, “4.58”]
> 
> What is the method I can use to total these?

You could use the class methods provided by ActiveRecord to do this,  
which will use the database’s aggregate features. This should also  
provide better performance.

total\_duration = Song.sum(:duration, :conditions =\> [ ‘album\_id = ?’,  
album ])

SQL: SELECT sum(“songs”.duration) AS sum\_duration FROM songs WHERE  
(album\_id = 1)

Personally, I would also add this as a method on Album for a nice clean  
API:

my\_song.duration  
=\> 4.05

my\_album.duration  
=\> 20.00

class Album \< ActiveRecord::Base  
def duration  
Song.sum(:duration, :conditions =\> [‘album\_id = ?’, self.id])  
end  
end

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<div class="post-metadata">

**Author:** ![mercurius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c89c15/32.png) [@mercurius](https://www.ruby-forum.com/u/mercurius)\
**Post date:** [December 17, 2009, 5:24pm UTC](https://www.ruby-forum.com/t/using-sum/180329/3 "2009-12-17T17:24:50Z")

</div>

Sorry, I wasn’t thinking at all. I’ve changed the duration to a time  
field now. I did have the field as a decimal with a precision of 5 and a  
scale of 2 but clearly this is not the way to measure total time.

What would you suggest for totalling time fields?

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<div class="post-metadata">

**Author:** ![mercurius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c89c15/32.png) [@mercurius](https://www.ruby-forum.com/u/mercurius)\
**Post date:** [December 18, 2009, 12:45am UTC](https://www.ruby-forum.com/t/using-sum/180329/4 "2009-12-18T00:45:40Z")

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Pale H. wrote:

> Sorry, I wasn’t thinking at all. I’ve changed the duration to a time  
> field now. I did have the field as a decimal with a precision of 5 and a  
> scale of 2 but clearly this is not the way to measure total time.

Why not?

> What would you suggest for totalling time fields?

I think I would suggest storing times in seconds in the DB and using  
composed\_of to convert. I think DB time types tend to be meant for time  
stamps, not time intervals. Of couse, if your DB has a time interval  
type, then use it!

Best,  
–Â  
Marnen Laibow-Koser  
[http://www.marnen.org](http://www.marnen.org)  
removed\_email\_address@domain.invalidÂ

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<div class="post-metadata">

**Author:** ![mercurius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c89c15/32.png) [@mercurius](https://www.ruby-forum.com/u/mercurius)\
**Post date:** [December 18, 2009, 7:45am UTC](https://www.ruby-forum.com/t/using-sum/180329/5 "2009-12-18T07:45:46Z")

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Marnen Laibow-Koser wrote:

> Pale H. wrote:
> 
> > Sorry, I wasn’t thinking at all. I’ve changed the duration to a time  
> > field now. I did have the field as a decimal with a precision of 5 and a  
> > scale of 2 but clearly this is not the way to measure total time.
> 
> Why not?

+1

Time fields are bad for storing time intervals.

> > What would you suggest for totalling time fields?
> 
> I think I would suggest storing times in seconds in the DB and using  
> composed\_of to convert. I think DB time types tend to be meant for time  
> stamps, not time intervals. Of couse, if your DB has a time interval  
> type, then use it!

Or milliseconds if you need more precision. Consider how UNIX tracks  
time, which is the number of milliseconds since January 1, 1970 stored  
in a “long” data type (32 bit integer). You wont really need that much  
for your case album durations aren’t going to be that long. Store the  
duration as a simple integer and format it for your views.

If you try to use time fields you’ll run into insane issues with  
timezone changes, daylight savings, etc. It just a bad idea.