title: Mongomapper querying on 'belongs_to' model
body: <p>I have two models:</p>
  
  <pre><code>class Post
   include MongoMapper::Document
  
   many :comments
  
   key :content, String
  end
  </code></pre>
  
  <p>and</p>
  
  <pre><code>class Comment
    include MongoMapper::Document
  
    belongs_to :post
  
    key :post_id, ObjectId
    key :content, String
  
  end
  </code></pre>
  
  <p>in a rails console session I can find all Posts:</p>
  
  <pre><code>Post.all # -&gt; [#&lt;Post _id: BSON::ObjectId('519b0b613…
  </code></pre>
  
  <p>and all comments associated with a Post:</p>
  
  <pre><code>post = Post.first # -&gt; #&lt;Post _id: BSON::ObjectId('519b0b613e477b…
  post.comments     # -&gt; [#&lt;Comment _id: BSON::ObjectId('519d14f93e…
  </code></pre>
  
  <p>however, the following query strangely returns an empty array</p>
  
  <pre><code>Comment.all # -&gt; []
  </code></pre>
  
  <p>Why? How can I get a list of all comments independently of the posts?</p>