From: Alex Gutteridge <alexg@...>
Date: 2007-04-26T14:23:00+09:00
Subject: Re: More concise code

On 26 Apr 2007, at 13:10, barjunk wrote:

> I found this snippet in code called acts_as_authenticated.  I thought
> it interesting but hard to understand at first.
>
>  @current_user ||= (session[:user] && User.find_by_id(session[:user]))
> || :false
>
> Now for some possible embarrassment.
>
> @current_user should return a value unless it is nil.

Perhaps I'm misunderstanding what you're saying here, but I think  
it's the opposite meaning. @current_user will be set to a new value  
(returned by the right hand side of the expression) unless it is  
already true. I.e. if it is false or nil it will be set to the RHS.  
Remember x ||= y expands to x = x || y

> If nil(false),
> then @current_user is set to session[:user] if session[:user] &&
> User.find_by_id return true
>
> otherwise, @current_user becomes false
>
> Am I even close?
>
> Mike B.

Almost. I'm not sure what the first line of your pseudo code means,  
but the second line is a little bit wrong. @current_user is set to  
the result of User.find_by_id(session[:user]), not session[:user].

Here's my expanded Ruby code interpretation:

unless @current_user #Unless we already have a user set
   if session[:user] and User.find_by_id(session[:user]) #Test if we  
have session user and id
     @current_user = User.find_by_id(session[:user]) #If we do then  
set user to id
   else
     @current_user = :false #other wise user is set to :false
   end
end

Some irb might help with all the ||=, ||, && line noise:

irb(main):004:0> foo = nil
=> nil
irb(main):005:0> bar = true
=> true
irb(main):006:0> foo ||= bar
=> true
irb(main):007:0> "foo" || "bar"
=> "foo"
irb(main):008:0> "foo" && "bar"
=> "bar"

Alex Gutteridge

Bioinformatics Center
Kyoto University