title: Trying to sort object keys based on child property
body: <p>I have this object:</p>
  
  <pre><code>var myObject = {
      cat: {
          order: 1
      },
  
      mouse: {
          order: 4
      },
  
      dog: {
          order: 2
      },   
  
      shark: {
          order: 3
      }
  }
  </code></pre>
  
  <p>I'm trying to get back: <code>["cat", "dog", "shark", "mouse"]</code></p>
  
  <p>I tried:</p>
  
  <p><code>_.sortBy(x, function(e) { return e.order} )</code></p>