title: Join two mongoid criteria
body: <p>I have an array with two  Mongoid::Criteria</p>
  
  <pre><code>[#&lt;Mongoid::Criteria
        selector: {"news_category_id"=&gt;"1"},
        options:  {:sort=&gt;[[:published_date, :desc]], :limit=&gt;1},
        class:    News,
        embedded: false&gt;
      , #&lt;Mongoid::Criteria
        selector: {"news_category_id"=&gt;"2"},
        options:  {:sort=&gt;[[:published_date, :desc]], :limit=&gt;1},
        class:    News,
        embedded: false&gt;
      ]
  </code></pre>
  
  <p>How can I get one Mongoid::Criteria object from that array of criteria?</p>
  
  <p>When that array rendered , it contains "Array of Array of json objects" and I want an array of json objects. (single merged array of json)</p>