How does mongoose populate an attribute field of an object array inside?

  node.js, question

When using Mongoose, I want to populate an object array field in User inside with an attribute Q of inside

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:}
 var User = new Schema({
 username: {type: String},
 password: {type: String},
 typeQuestion: [{q: {type: Schema.Types.ObjectId, ref: 'Question'}, a: String,option:String }],

Does anyone know how to do this