# \[Bug\] !nil returns false for partial instance variable in specific conditions **URL:** \ **Category:** Rails\ **Created:** [December 7, 2008, 6:36am UTC](https://www.ruby-forum.com/t/bug-nil-returns-false-for-partial-instance-variable-in-specific-conditions/155061 "2008-12-07T06:36:15Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![vikrant](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/cc9497/32.png) [@vikrant](https://www.ruby-forum.com/u/vikrant)\ **Post date:** [December 7, 2008, 6:36am UTC](https://www.ruby-forum.com/t/bug-nil-returns-false-for-partial-instance-variable-in-specific-conditions/155061/1 "2008-12-07T06:36:15Z")
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 - ``` identity is <%= identity.inspect %> !identity is <%= (!identity).inspect %> ``` 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](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](http://code.google.com/p/googol/source/browse/?r=95#svn/lab/learning/ruby-bug-on-!nil/app/controllers) - thanks ---
**Author:** ![vikrant](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/cc9497/32.png) [@vikrant](https://www.ruby-forum.com/u/vikrant)\ **Post date:** [December 7, 2008, 6:20pm UTC](https://www.ruby-forum.com/t/bug-nil-returns-false-for-partial-instance-variable-in-specific-conditions/155061/2 "2008-12-07T18:20:14Z")
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). On Dec 7, 3:13 pm, Frederick C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![vikrant](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/cc9497/32.png) [@vikrant](https://www.ruby-forum.com/u/vikrant)\ **Post date:** [December 7, 2008, 11:14am UTC](https://www.ruby-forum.com/t/bug-nil-returns-false-for-partial-instance-variable-in-specific-conditions/155061/3 "2008-12-07T11:14:00Z")
On Dec 7, 5:35 am, Vikrant [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > 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 > 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 ---
**Author:** ![vikrant](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/cc9497/32.png) [@vikrant](https://www.ruby-forum.com/u/vikrant)\ **Post date:** [December 7, 2008, 6:35pm UTC](https://www.ruby-forum.com/t/bug-nil-returns-false-for-partial-instance-variable-in-specific-conditions/155061/4 "2008-12-07T18:35:29Z")
On 7 Dec 2008, at 17:19, Vikrant wrote: > 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 > It took me 4+ hours to figure out what went wrong and where (I’m not > complaining though).