title: How to define a generic nested object in Mongoose
body: <p>I would like to have a nested object in the detail for the activiy log. See the example. How do I define the schema in mongoose?</p>
  
  <pre><code>activity: {
      date: '1/1/2012' ,
      user: 'benbittly', 
      action: 'newIdea', 
      detail: {
          'title': 'how to nest'
          , 'url': '/path/to/idea'
      }
  
  activity: {
      date: '1/2/2012' ,
      user: 'susyq', 
      action: 'editProfile', 
      detail: {
          'displayName': 'Susan Q'
          , 'profileImageSize': '32'
          , 'profileImage': '/path/to/image'
      }
  </code></pre>