# Problem caching Model instances on a constant in Rails

**URL:** <https://www.ruby-forum.com/t/problem-caching-model-instances-on-a-constant-in-rails/167327>\
**Category:** Rails\
**Created:** [May 6, 2009, 7:24pm UTC](https://www.ruby-forum.com/t/problem-caching-model-instances-on-a-constant-in-rails/167327 "2009-05-06T19:24:48Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![fcruxen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/ecae2f/32.png) [@fcruxen](https://www.ruby-forum.com/u/fcruxen)\
**Post date:** [May 6, 2009, 7:24pm UTC](https://www.ruby-forum.com/t/problem-caching-model-instances-on-a-constant-in-rails/167327/1 "2009-05-06T19:24:48Z")

</div>

I am using Single-Table Inheritance (STI) on one of my models for a  
Rails App and I am having problems storing model objects on a constant.  
I have isolated the problem into a sample project and submitted it to  
GitHub: [GitHub - fcoury/rails-sti-caching: Isolated test for Rails Single-Table Inheritance caching problem](http://github.com/fcoury/rails-sti-caching)

What I am trying to do is loading a model instance (in this case a Music  
model, that inherits from the Media model via STI) on an initializer (in  
Rails’ `/config/initializers/` directory) and keep it on a constant:

```
MUSIC_CACHE = Hash.new
Music.all.each { |m| MUSIC_CACHE[m.id] = m }

```

And I have a sample controller that does the following:

```
class MusicsController < ApplicationController
  def index
    require 'pp'
    pp MUSIC_CACHE
    @debug = []
    MUSIC_CACHE.each_pair do |k, v|
      music = Music.find(k)
      d "Object for Music.find(#{k}) => class: #{music.class} -

```

class obj\_id: #{music.class.object\_id} - #{music.inspect}"  
d “Object for MUSIC\_CACHE[#{k}] =\> class: #{v.class} - class  
obj\_id: #{v.class.object\_id} - #{v.inspect}”

```
      begin
        d " - Music.is_a?(Media) => #{v.is_a?(Media)}"
        d " - Try to call name => #{v.name}"
      rescue
        d "*** Error raised:\n#{$!}"
      end
    end

    @musics = Music.all
  end

  def d(s)
    puts s
    @debug << s
  end
end

```

And a view to go with it:

```
<h1 id="music">Music</h1>

<ul>
  <% for m in @musics %>
  <li><%= m.name %> - <%= m.file %></li>
  <% end %>
</ul>

<pre><%=h @debug.join("\n") %></pre>

```

The first time this code runs, the output on the `<pre>` tag is this:

```
  Object for Music.find(2) => class: Music - class obj_id: 13067420

```

- #\<Music id: 2, name: “5th Symphony”, file: “5s.mp3”, type: “Music”,  
created\_at: “2009-05-06 16:31:41”, updated\_at: “2009-05-06 16:31:41”\>  
Object for MUSIC\_CACHE[2] =\> class: Music - class obj\_id: 13067420
- #\<Music id: 2, name: “5th Symphony”, file: “5s.mp3”, type: “Music”,  
created\_at: “2009-05-06 16:31:41”, updated\_at: “2009-05-06 16:31:41”\>  
- Music.is\_a?(Media) =\> true  
- Try to call name =\> 5th Symphony

However, if I just reload the page, here’s what gets outputted:

```
Object for Music.find(2) => class: Music - class obj_id: 18452280 -

```

#\<Music id: 2, name: “5th Symphony”, file: “5s.mp3”, type: “Music”,  
created\_at: “2009-05-06 16:31:41”, updated\_at: “2009-05-06 16:31:41”\>  
Object for MUSIC\_CACHE[2] =\> class: Music - class obj\_id: 13067420 -  
#\<Music id: 2, name: “5th Symphony”, file: “5s.mp3”, type: “Music”,  
created\_at: “2009-05-06 16:31:41”, updated\_at: “2009-05-06 16:31:41”\>  
- Music.is\_a?(Media) =\> false  
\*\*\* Error raised:  
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.include?

Does anyone know the rationale behind this error?

---

<div class="post-metadata">

**Author:** ![fcruxen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/ecae2f/32.png) [@fcruxen](https://www.ruby-forum.com/u/fcruxen)\
**Post date:** [May 6, 2009, 11:53pm UTC](https://www.ruby-forum.com/t/problem-caching-model-instances-on-a-constant-in-rails/167327/2 "2009-05-06T23:53:21Z")

</div>

at first looks like the old problem of rails reloading  
the classes and not recognizing the objects in memory  
anymore. try set “config.cache\_classes” to true in your  
development.rb file or run in production mode.