# Inject error when no default value is given?

**URL:** <https://www.ruby-forum.com/t/inject-error-when-no-default-value-is-given/102672>\
**Category:** Rails\
**Created:** [February 2, 2007, 8:42pm UTC](https://www.ruby-forum.com/t/inject-error-when-no-default-value-is-given/102672 "2007-02-02T20:42:34Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dfg59](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/e0b2c6/32.png) [@dfg59](https://www.ruby-forum.com/u/dfg59)\
**Post date:** [February 2, 2007, 8:42pm UTC](https://www.ruby-forum.com/t/inject-error-when-no-default-value-is-given/102672/1 "2007-02-02T20:42:34Z")

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I have an Event model which has\_many :attendees. My Attendee model has a  
column called participants, which is an integer representing the number  
of participants in the attendees party. I want to sum the total number  
of participants for a given event. This works fine if I just the default  
option of 0 for the method inject, but inject fails if you don’t give it  
a default. In ruby, injects normal behavior will default the sum to 0 if  
you don’t provide an argument. See my console output below:

> > myevent = Event.find(1)  
> > =\> #\<Event:0x34d95ec @attributes={“title”=\>“Canned Food Drive”,  
> > “date”=\>“2007-02-01 12:00:00”, “id”=\>“1”, “description”=\>“We need a  
> > least 20 people to sign up for the canned food drive. This will could  
> > toward service hours and will be a great event to help younger scouts  
> > get involved with the community. We will be collecting, sorting and  
> > delivering donated cans.”, “rsvp\_date”=\>“2007-01-15”,  
> > “permission\_url”=\>"/docs/permission.jpg", “location”=\>“Saint David’s”}\>
> 
> > myevent.attendees.inject(0){|sum,att| sum = att.participants}  
> > =\> 1
> 
> > myevent.attendees.inject{|sum,att| sum = att.participants}  
> > =\> #\<Attendee:0x34cc7ac @attributes={“name”=\>“Drew O.”,  
> > “event\_id”=\>“1”, “participants”=\>“1”, “contact”=\>“847.942.2606”,  
> > “id”=\>“1”}\>

As you can see, if a default of 0 is not provided, inject returns an  
instance of the Attendee class rather than the value in sum. Very  
strange…

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**Author:** ![dfg59](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/e0b2c6/32.png) [@dfg59](https://www.ruby-forum.com/u/dfg59)\
**Post date:** [February 2, 2007, 8:51pm UTC](https://www.ruby-forum.com/t/inject-error-when-no-default-value-is-given/102672/2 "2007-02-02T20:51:12Z")

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> As you can see, if a default of 0 is not provided, inject returns an  
> instance of the Attendee class rather than the value in sum. Very  
> strange…

My inject statements should have read inject{|sum,att| sum +  
att.participants}. I’ve rerun the code, now I am getting a full-fledged  
error when I don’t provide a default value:

> > myevent.attendees.inject{|sum,att| sum + att.participants}  
> > NoMethodError: undefined method `+' for #<Attendee:0x34cca90> from /opt/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4.5618/lib/active_record/base.rb:1848:in`method\_missing’  
> > from (irb):2  
> > from  
> > /opt/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4.5618/lib/active\_record/base.rb:1928:in  
> > `inject' from /opt/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4.5618/lib/active_record/associations/association_proxy.rb:123:in`each’  
> > from  
> > /opt/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4.5618/lib/active\_record/associations/association\_proxy.rb:123:in  
> > `inject' from /opt/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4.5618/lib/active_record/associations/association_proxy.rb:123:in`send’  
> > from  
> > /opt/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4.5618/lib/active\_record/associations/association\_proxy.rb:123:in  
> > `method_missing' from /opt/local/lib/ruby/gems/1.8/gems/activerecord-1.14.4.5618/lib/active_record/associations/has_many_association.rb:98:in`method\_missing’  
> > from (irb):2
> 
> > myevent.attendees.inject(0){|sum,att| sum + att.participants}  
> > =\> 5

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<div class="post-metadata">

**Author:** ![dfg59](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/e0b2c6/32.png) [@dfg59](https://www.ruby-forum.com/u/dfg59)\
**Post date:** [February 2, 2007, 9:07pm UTC](https://www.ruby-forum.com/t/inject-error-when-no-default-value-is-given/102672/3 "2007-02-02T21:07:19Z")

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On Feb 2, 2:51 pm, Drew O. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> NoMethodError: undefined method `+' for #<Attendee:0x34cca90> from `method\_missing’  
> from (irb):2\>\> myevent.attendees.inject(0){|sum,att| sum + att.participants}
> 
> =\> 5
> 
> –  
> Posted viahttp://www.ruby-forum.com/.

IIRC,

Calling inject with no parameter copies the first element of the  
enumerable into the accumulator. In this case it fills it with the  
first attendee. It never calls ‘participants’ on it.

This works fine when you are using it on numbers, but runs into  
trouble with more complicated data types.

Doing this would probably work as you expect.

myevent.attendees.map(&:participants).inject {|sum,att| sum + att}

\_Kevin