title: about mongoose populate relation a string field
body: <p>I have two Schema</p>
  
  <p>1 - User</p>
  
  <pre><code>UserSchema = new db.Schema({
      email:  {type: String, required: true},
      pass:   {type: String, required: true},
      nick:   {type: String, required: true},
      admin:  {type: String, default: ''},
      reg:    {type: Date, default: Date.now}
  });
  </code></pre>
  
  <p>2 - Article</p>
  
  <pre><code>ArticleSchema = new db.Schema({
      title:      {type: String, required: true},
      alise:      {type: String},
      time:       {type: Date, defaults: Date.now},
      view:       {type: Number, defaults: 0},
      author:     {type: String},
      content:    {type: String},
      classes:    {type: db.Schema.Types.ObjectId, ref: 'Classes'},
      user:       {type: String, ref: 'User'}
  });
  </code></pre>
  
  <p>I want ArticleSchema user field relation UserSchema nick.</p>
  
  <p>my code:</p>
  
  <pre><code>Model.findOne({}).populate('classes user').exec(function(err, res){
      if(err){
          cb(err);
      }else{
          cb(null, res);
      }
  });
  </code></pre>
  
  <p>This is not working</p>
  
  <pre><code>message: 'Cast to ObjectId failed for value "tudou" at path "_id"'
  </code></pre>
  
  <p>What should I do?  </p>