case：

// mongoose.model('User', UserSchema);      
var mongoose = require('mongoose'),
  Schema = mongoose.Schema;
  var UserSchema = new Schema({
  name: {
  type: String,
  unique: 'Username already exists',
  required: 'Please fill in a username',
  trim: true   },
  group:  {
  type: Schema.Types.ObjectId ,
  ref: 'Group'
  }
  ……
  });
  // mongoose.model('Group', UserSchema);       
  var GroupSchema = new Schema({
  name: {
  type: String,
  unique: 'Groupname already exists',
  required: 'Please fill in a Groupname',
  trim: true   },
  collection:  {
  type: Schema.Types.ObjectId ,
  ref: 'Collection'
  }
  ……
  });
    // mongoose.model('Collection', UserSchema);        
var CollectionSchema = new Schema({
  title: {
  type: String,
  trim: true   }
  ……
  });
Q:User.findOne().poplation('group').exec(function(..){
可以获得群组的信息，但怎样通过这样的查询直接获得collection的title信息等？
})