title: mongoid find VS where
body: <p>i seem to only be having this issue with 1 particular model when i am searching by ID</p>
  
  <pre><code>&gt;&gt; Cart.where(:_id =&gt; '4dae5902e1607c232c000009').first
  =&gt; #&lt;Cart _id: 4dae5902e1607c232c000009, _id: BSON::ObjectId('4dae5902e1607c232c000009'), _type: nil&gt;
  &gt;&gt; Cart.find('4dae5902e1607c232c000009')
  Mongoid::Errors::DocumentNotFound: Document not found for class Cart with id(s) 4dae5902e1607c232c000009.
  </code></pre>
  
  <p>the strange thing is that with other models, i can use find just fine.  any ideas?</p>
  
  <p>the rest of the stack is...</p>
  
  <pre><code>from /Library/Ruby/Gems/1.8/gems/mongoid-2.0.1/lib/mongoid/criterion/inclusion.rb:192:in `execute_or_raise'
  from /Library/Ruby/Gems/1.8/gems/mongoid-2.0.1/lib/mongoid/criterion/inclusion.rb:190:in `tap'
  from /Library/Ruby/Gems/1.8/gems/mongoid-2.0.1/lib/mongoid/criterion/inclusion.rb:190:in `execute_or_raise'
  from /Library/Ruby/Gems/1.8/gems/mongoid-2.0.1/lib/mongoid/criterion/inclusion.rb:106:in `find'
  from /Library/Ruby/Gems/1.8/gems/mongoid-2.0.1/lib/mongoid/finders.rb:67:in `find'
  from (irb):37
  </code></pre>