title: keys must be strings or symbols
body: <p>Setup:</p>
  
  <pre><code>Ruby on rails 3.2.2
  Ruby 1.9.2
  dataMapper 1.2.0
  mongoid 2.4.8
  mongoid_orderable 1.0.0
  </code></pre>
  
  <p>Getting <code>keys must be strings or symbols</code> error when using mongoid_orderable.</p>
  
  <p>I think it has something to do with dataMapper which I am using besides mongid (porting data from anoher server with dataMapper)</p>
  
  <p>I've notices collisions between dataMapper and mongoid, since they both extend Symbol with various methods, asc, desc etc. Could this be the same problem?</p>
  
  <p>Here is a snippet from my stacktrace if it is any help:</p>
  
  <pre><code>keys must be strings or symbols
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/bson-1.6.2/lib/bson/bson_c.rb:24:in `serialize'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/bson-1.6.2/lib/bson/bson_c.rb:24:in `serialize'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongo-1.6.2/lib/mongo/collection.rb:436:in `update'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongoid-2.4.8/lib/mongoid/collections/master.rb:25:in `block in update'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongoid-2.4.8/lib/mongoid/collections/retry.rb:29:in `retry_on_connection_failure'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongoid-2.4.8/lib/mongoid/collections/master.rb:24:in `update'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongoid-2.4.8/lib/mongoid/collection.rb:149:in `update'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongoid_orderable-1.0.0/lib/mongoid_orderable/mongoid/contexts/mongo.rb:6:in `inc'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongoid_orderable-1.0.0/lib/mongoid_orderable/mongoid/criteria.rb:1:in `inc'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongoid_orderable-1.0.0/lib/mongoid/orderable.rb:125:in `apply_position'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/mongoid_orderable-1.0.0/lib/mongoid/orderable.rb:85:in `add_to_list'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:419:in `block in _run__3008157942946527494__save__4591629889417243504__callbacks'
  /Users/hmm/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/activesupport-3.2.2/lib/active_support/callbacks.rb:215:in `block in _conditional_callback_around_742'
  </code></pre>
  
  <p>Current implementation of mongoid_orderable is following:</p>
  
  <pre><code>class MyModel
    include Mongoid::Document
    include Mongoid::Orderable
    default_scope order_by(:position =&gt; :asc)
  end
  </code></pre>
  
  <p>I'm pretty sure it is related to _mongoid_orderable_ based on stacktrace and the fact that it works if I comment out _mongoid_orderable_ related lines. Also filed an issue <a href="https://github.com/pyromaniac/mongoid_orderable/issues/4">here</a></p>