title: rails friendly_id and check if entry exists
body: <p>How to check if <a href="https://github.com/norman/friendly_id">friendly_id</a> entry exists before get it? </p>
  
  <p>For example:</p>
  
  <pre><code>  def book
     @book = Book.find(params[:book_id])
    end
  </code></pre>
  
  <p>It's ok, but i want check before if friendly_id exists, something like:</p>
  
  <pre><code>def book
    if Book.exists?(params[:book_id])
      @book = Book.find(params[:book_id])
    else
      #404 not found
    end
  </code></pre>