title: Duplicate key error index on embedded document in mongoose
body: <p>I am trying to run this simple embedded document using mongoose:</p>
  
  <pre><code>var mongoose = require('mongoose');
  var PageSchema = new mongoose.Schema({
      url:String
  });
  var AlbumSchema = new mongoose.Schema({
      pages:[ PageSchema ]
  });
  
  mongoose.model('Album', AlbumSchema);
  var Album = mongoose.model('Album');
  var album = new Album({pages:[{url:"1"}]});
  album.save(function(err, a) {
      console.log(err);
  });
  </code></pre>
  
  <p>After I run this code the second time I get this Error:</p>
  
  <pre><code>{ 
      [MongoError: E11000 duplicate key error index: doalbums.albums.$pages.id_1  dup key: { : null }]
      name: 'MongoError',
      err: 'E11000 duplicate key error index: doalbums.albums.$pages.id_1  dup key: { : null }',
      code: 11000,
      n: 0,
      connectionId: 161,
      ok: 1 
  }
  </code></pre>
  
  <p>What am I doing wrong?</p>