title: Mongoid where vs find when using id as criteria
body: <p>when using document's id as the only criteria in my query what's the difference between:</p>
  
  <pre><code>Board.only(:_id).find(params[:board_id])
  </code></pre>
  
  <p>and </p>
  
  <pre><code>Board.where(_id: params[:board_id]).only(:_id)
  </code></pre>
  
  <p>the only thing i've noticed is that printing the result as json when using where it encloses the result in square brackets</p>