title: return all fields of document without specified fields in model
body: <p>I want to show all fields of returning documents.</p>
  
  <p>Because the returned docuemets' fields are differed.</p>
  
  <p>I want to show all the fields without <code>specifying</code> fields in the model.</p>
  
  <h1>Model</h1>
  
  <pre><code>class History
    include Mongoid::Document
  end
  </code></pre>
  
  <h1>Returned documents only contains <code>_id</code> for now</h1>
  
  <pre><code>+--------------------------+
  | _id                      |
  +--------------------------+
  | 558a64b9253c9b33cbc90f8c |
  | 558a64b9253c9b33cbc90f8d |
  | 558a64b9253c9b33cbc90f8e |
  | 558a64b9253c9b33cbc90f8f |
  | 558a64b9253c9b33cbc90f90 |
  | 558a64b9253c9b33cbc90f91 |
  | 558a64b9253c9b33cbc90f92 |
  | 558a64b9253c9b33cbc90f93 |
  | 558a64b9253c9b33cbc90f94 |
  | 558a64b9253c9b33cbc90f95 |
  +--------------------------+
  </code></pre>