title: mongoose populate a filed in a subdocument array
body: <p>With mongoose, I want to populate a property inside a subdocument Array of my main document.  Here is example:</p>
  
  <pre><code>var Question = new Schema({
    title: {type: String},
    answers: Schema.Types.Mixed, //{A:'',B:''}
    category: {type: String}, 
    type: {type: String},        
    enable: {type: Boolean, default: true},
    sex: Number,             
    //random: {type: Number, default: Math.random}, 
    createdAt: {type: Date, default: Date.now} 
  });
  
  var User = new Schema({
    username: {type: String},    
    password: {type: String},
    typeQuestion: [{q: {type: Schema.Types.ObjectId, ref: 'Question'}, a: String,option:String }],    
  });
  </code></pre>
  
  <p>How to populate typeQuestion from User, Anyone can tell me how to do it?</p>