Non _id document reference

test.js

var mongoose = require('mongoose');
var db = mongoose.createConnection('localhost', 'test');

var UserSchema = mongoose.Schema({
  name: String,
  phone: Number
});

var MemorySchema = mongoose.Schema({
  user: { type: Number, ref: 'User' },
  text: String
});

var User = db.model('User', UserSchema);
var Memory = db.model('Memory', MemorySchema);

User.create({ name: "bhelx", phone: 15556667788 }, function (err, u) {
  Memory.create({ text: "Hello World!", user: u.phone }, function (err, m) {
    Memory.findById(m.id).populate('user').exec(function (err, memory) {
      console.log(err);
      console.log(memory);
    });
  });
});