# Explanation of method

**URL:** <https://www.ruby-forum.com/t/explanation-of-method/119225>\
**Category:** Rails\
**Created:** [November 5, 2007, 6:58am UTC](https://www.ruby-forum.com/t/explanation-of-method/119225 "2007-11-05T06:58:42Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Chubbs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/47e85d/32.png) [@Chubbs](https://www.ruby-forum.com/u/Chubbs)\
**Post date:** [November 5, 2007, 6:58am UTC](https://www.ruby-forum.com/t/explanation-of-method/119225/1 "2007-11-05T06:58:42Z")

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hey i have a method

def admin\_login\_required  
username, passwd = get\_auth\_data  
self.current\_user ||= User.authenticate(username, passwd)  
|| :false if username && passwd  
logged\_in? && authorized? ? true : access\_denied  
end

this is from an authentication plugin. i cant make sense of it, and  
dont understand wot the ‘||=’ symbol does. i cant google for it, cos  
google strips the search for the symbol. can someone explain step by  
step wot the function is doing ?

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**Author:** ![Chubbs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/47e85d/32.png) [@Chubbs](https://www.ruby-forum.com/u/Chubbs)\
**Post date:** [November 5, 2007, 11:23am UTC](https://www.ruby-forum.com/t/explanation-of-method/119225/2 "2007-11-05T11:23:05Z")

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On 5 Nov 2007, at 05:57, Chubbs wrote:

> this is from an authentication plugin. i cant make sense of it, and  
> dont understand wot the ‘||=’ symbol does. i cant google for it, cos  
> google strips the search for the symbol. can someone explain step by  
> step wot the function is doing ?

This is a ruby question so probably better suited to other places, but  
anyway,  
for most operators  
a op= b is the same as a = a op b  
So a ||= b is the same a = a || b  
Because of the way ruby evaluates these things, it means set a to b  
unless a is already set (in which case it won’t even evaluate b).

Fred

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**Author:** ![Chubbs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/47e85d/32.png) [@Chubbs](https://www.ruby-forum.com/u/Chubbs)\
**Post date:** [November 5, 2007, 11:37am UTC](https://www.ruby-forum.com/t/explanation-of-method/119225/3 "2007-11-05T11:37:10Z")

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Hi –

On Mon, 5 Nov 2007, Frederick C. wrote:

> > || :false if username && passwd  
> > This is a ruby question so probably better suited to other places, but  
> > anyway,  
> > for most operators  
> > a op= b is the same as a = a op b  
> > So a ||= b is the same a = a || b  
> > Because of the way ruby evaluates these things, it means set a to b  
> > unless a is already set (in which case it won’t even evaluate b).

There’s at least one edge-case which reveals that it a ||= b and  
a = a || b aren’t quite the same:

irb(main):007:0\> h = Hash.new(1)  
=\> {}  
irb(main):008:0\> h[:x] ||= 2  
=\> 1  
irb(main):009:0\> h  
=\> {}

Here’s what happens with the plain || version:

irb(main):010:0\> h[:y] = h[:y] || 3  
=\> 1  
irb(main):011:0\> h  
=\> {:y=\>1}

Matz had something to say about ||= at the Ruby Clinic that I  
conducted on Friday at RubyConf – namely, that it’s better to think  
of it this way:

x ||= y =\> x || (x = y)

Translating my example that way produces the right result.

David

–  
Upcoming training by David A. Black/Ruby Power and Light, LLC:

- Advancing With Rails, Edison, NJ, November 6-9
- Advancing With Rails, Berlin, Germany, November 19-22
- Intro to Rails, London, UK, December 3-6 (by Skills Matter)  
See [http://www.rubypal.com](http://www.rubypal.com) for details!

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**Author:** ![Chubbs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/47e85d/32.png) [@Chubbs](https://www.ruby-forum.com/u/Chubbs)\
**Post date:** [November 5, 2007, 12:09pm UTC](https://www.ruby-forum.com/t/explanation-of-method/119225/4 "2007-11-05T12:09:54Z")

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On 5 Nov 2007, at 10:36, David A. Black wrote:

> Matz had something to say about ||= at the Ruby Clinic that I  
> conducted on Friday at RubyConf – namely, that it’s better to think  
> of it this way:
> 
> x ||= y =\> x || (x = y)

You learn something everyday! Thanks for that.

Fred