title: How to $set sub-sub-array items in MongoDB
body: <p>I'm developing a webapp which has a portal-ish component to it (think like multiple panels that can be drug around from column to column and added or removed).  I'm using MongoDB to store this info with a format like so...</p>
  
  <pre><code>{
      _id: ObjectId(...),
      title: 'My Layout',
      columns: [
          {
              order: 1,
              width: 30,
              panels: [
                  { title: 'Panel Title', top: 100, content: '...' },
                  { title: 'Panel Title', top: 250, content: '...' },
              ]
          },
          {
              ... multiple columns ...
          }
      ]
  }
  </code></pre>
  
  <p>I'm attempting to use atomic/modifier operations with <strong>update()</strong> and this is getting confusing.  If I wanted to just update one property of a specific panel, how do I reference that?</p>
  
  <pre><code>update(
      { _id: ObjectId(...) },
      { $set: { columns.[???].panels.[???].top: 500 }
  )
  </code></pre>