# Attr\_accessor\_with\_default funny

**URL:** <https://www.ruby-forum.com/t/attr-accessor-with-default-funny/137140>\
**Category:** Rails\
**Created:** [May 9, 2008, 10:29am UTC](https://www.ruby-forum.com/t/attr-accessor-with-default-funny/137140 "2008-05-09T10:29:10Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![matthew](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ed655f/32.png) [@matthew](https://www.ruby-forum.com/u/matthew)\
**Post date:** [May 9, 2008, 10:29am UTC](https://www.ruby-forum.com/t/attr-accessor-with-default-funny/137140/1 "2008-05-09T10:29:10Z")

</div>

I created some virtual attributes in a model in order to store  
intermediate calculated values. I wanted to use  
attr\_accessor\_with\_default so that I could give the calculated values  
an initial value to allow me to use += to easily accumulate values.  
When I did so I came across some slightly perplexing behaviour which I  
was wondering if anybody could explain to me.

When I tried to create a method in the class which used += on the bare  
attribute it complained that the attribute was nil

D:\Data\Ruby\footy\>ruby script\console  
Loading development environment (Rails 2.0.2)

> > class Foo  
> > attr\_accessor\_with\_default :bar, 0  
> > def increment\_bar  
> > bar += 1  
> > end  
> > end  
> > =\> nil
> 
> > f = Foo.new  
> > =\> #Foo:0x37b47d0
> 
> > f.increment\_bar  
> > NoMethodError: You have a nil object when you didn’t expect it!  
> > You might have expected an instance of Array.  
> > The error occurred while evaluating nil.+  
> > from (irb):4:in `increment\_bar’  
> > from (irb):8

On the other hand, if I referred to the attribute using the self  
qualifier, everything worked fine.

D:\Data\Ruby\footy\>ruby script\console  
Loading development environment (Rails 2.0.2)

> > class Foo  
> > attr\_accessor\_with\_default :bar, 0  
> > def increment\_bar  
> > self.bar += 1  
> > end  
> > end  
> > =\> nil
> 
> > f = Foo.new  
> > =\> #Foo:0x37b43c0
> 
> > f.increment\_bar  
> > =\> 1

If someone is able to explain this in words of one syllable that would  
be greatly appreciated.

---

<div class="post-metadata">

**Author:** ![matthew](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ed655f/32.png) [@matthew](https://www.ruby-forum.com/u/matthew)\
**Post date:** [May 9, 2008, 11:13am UTC](https://www.ruby-forum.com/t/attr-accessor-with-default-funny/137140/2 "2008-05-09T11:13:29Z")

</div>

Hi –

On Fri, 9 May 2008, Matthew wrote:

> =\> #Foo:0x37b47d0  
> D:\Data\Ruby\footy\>ruby script\console
> 
> > > f.increment\_bar  
> > > =\> 1
> 
> If someone is able to explain this in words of one syllable that would  
> be greatly appreciated.

When you go like this:

x = 1

the tongue by Matz thinks you want to set x to 1. And this:

x += 1 # change x to x plus 1

means (more or less) this:

x = x + 1

If you have a thing you can call (you know – one of those things you  
write with “def”), like this:

def x=(n)

and you want to run it, you have to use self (or, if not self, an apt  
thing, like y or z or what have you):

self.x = 1  
y.x = 2

so that the tongue by Matz knows you want to call that thing. It’s a  
case of “It could be this, or it could be that; please make it clear,  
or I will fall back on the ‘set x to n’ view.”

David

–  
Rails training from David A. Black and Ruby Power and Light:  
INTRO TO RAILS June 9-12 Berlin  
ADVANCING WITH RAILS June 16-19 Berlin  
INTRO TO RAILS June 24-27 London (Skills Matter)  
See [http://www.rubypal.com](http://www.rubypal.com) for details and updates!

---

<div class="post-metadata">

**Author:** ![matthew](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ed655f/32.png) [@matthew](https://www.ruby-forum.com/u/matthew)\
**Post date:** [May 9, 2008, 11:13am UTC](https://www.ruby-forum.com/t/attr-accessor-with-default-funny/137140/3 "2008-05-09T11:13:29Z")

</div>

On May 9, 9:28 am, Matthew [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> =\> nil
> 
> > > f = Foo.new

When you do bar+= 1, ruby thinks you are talking about the local  
variable bar. When you do self.bar += 1, ruby knows that you’re  
talking about the method (you haven’t defined a bar local variable but  
saying bar += 1 causes one (with value nil) to spring into existance;  
ruby local variable stuff is a little funky in places.

Fred