title: mongoose schema multi ref for one property
body: <p>How to write multi ref for one property of one mongoose schema, like this(but wrong):</p>
  
  <pre><code>var Schema = mongoose.Schema;
  var PeopleSchema = new Schema({
      peopleType:{
          type: Schema.Types.ObjectId,
          ref: ['A', 'B'] /*or 'A, B'*/
      }
  })
  </code></pre>