title: mongoose populate field without ref option
body: <p>I have 3 schema:</p>
  
  <pre><code>var User1Schema = new Schema({
      name: {type:String , trim: true, required: true },
      status: {type:Number, required: true, default:1}
  },{collection:"user_1"});
  </code></pre>
  
  <p>,</p>
  
  <pre><code>var User2Schema = new Schema({
      name: {type:String , trim: true, required: true },
      email: {type: String, required: true, index: {unique: true} },
      status: {type:Number, required: true, default:1}
  },{collection:"user_2"});
  </code></pre>
  
  <p>and </p>
  
  <pre><code>var ConversationSchema = new Schema( {
  
      participants: [{user:{type: ObjectId, required: true}],
      creator: {type: ObjectId, required: true},
  
      status: { type:Number, required: true, default:1 }
  
  }, { collection:"conversation" } );
  </code></pre>
  
  <p>In ConversationSchema I have creator field whic has now ref option, because it can be type of User1Schema or User2Schema.</p>
  
  <p>How can I populate creator field using mongoose</p>