!nil returns false for partial instance variable in specific conditions.

rubyonrails-talk
1

Hi, If we call a partial with a name such that there also exists a instance variable with the same name (ex: @identity) in the controller then passing :object => nil to render() and calling (! identity).inspect in '_identity.html.erb' prints false while identity.inspect prints nil.

Here is a more specific description -

Let there be a controller named Check.

class CheckController < ApplicationController   def index     @identity = nil   end end

views/check/index.html.erb contains - <%= render :partial => 'identity', :object => nil %>

views/check/_identity.html.erb contains - <pre>   identity is <%= identity.inspect %>   !identity is <%= (!identity).inspect %> </pre>

calling localhost:3000/check/ prints -   identity is nil   !identity is false

NOTE: 1. There must be a controller instance variable with the same name as of partial. 2. I have been able to trace the problem down to between line 31 and 42 of file http://github.com/rails/rails/tree/dc0411fad78bfc92fe92dc88bbad726eb4d1a883/actionpack/lib/action_view/renderable_partial.rb

I'm using - Rails 2.2.2 Ruby 1.8.6 (2007-09-14 patchlevel 111) [i386-mswin32] RubyGems 1.3.1 Windows XP Home Ed. SP3

I've also uploaded a sample app folder at http://code.google.com/p/googol/source/browse/?r=95#svn/lab/learning/ruby-bug-on-!nil/app/controllers

- thanks

2

Hi, If we call a partial with a name such that there also exists a instance variable with the same name (ex: @identity) in the controller then passing :object => nil to render() and calling (! identity).inspect in '_identity.html.erb' prints false while identity.inspect prints nil.

The key here is Deprecated implicit local assignments when rendering partials · rails/rails@acbf2b7 · GitHub

What is being deprecated is render :partial => 'foo' automatically taking the value of @foo. The way that this is done is that the foo object in your partial is actually a tiny proxy object. If you try and do anything with that object it will just pass it on to the real object (and print a small deprecation notice - you should see these in your logs). Ruby being ruby, ==, inspect, class etc... can all be passed on to the target objects - they are just method calls after all. That is not true of ! though, which just flips nil/false to true and everything else to false and so you get the behaviour you demonstrate.

Fred

3

Yes, that might be the "reason" (though not technically strong enough to understand that proxy thing), but still I find it as a bug. It just breaks the common assumptions (assumption is perhaps wrong word, it breaks just how it must work). nil is after all "not true" (and so is ! nil). It took me 4+ hours to figure out what went wrong and where (I'm not complaining though).

4

Yes, that might be the "reason" (though not technically strong enough to understand that proxy thing), but still I find it as a bug. It just breaks the common assumptions (assumption is perhaps wrong word, it breaks just how it must work). nil is after all "not true" (and so is ! nil).

Short version is that the thing that looks like nil isn't actually nil What it's trying to tell you is that having @identity magically assigned to identity in partials called identity is deprecated

Fred