反向填充猫鼬

我该如何反向填充mongo.我有2个架构的

How can i reverse populate in mongo. I have 2 schema's

用户:

var user_scheme = new mongoose.Schema({
  name:String,
  age:Number,
  roles:{
    type:mongoose.Schema.Types.ObjectId,
    ref:'Role'
  }
 });

角色:

var role_scheme = new mongoose.Schema({
  name:String,
});

文档:

///user
{
"_id" : ObjectId("5bf9a19b01ce3e19b440aed8"),
"name" : "user1",
"age" : 22,
"roles" : ObjectId("5c0242621ab7b677e6b2e01e"),
"__v" : 0
}
 ///role
{
"_id" : ObjectId("5c0242621ab7b677e6b2e01e"),
"name" : "Admin"
}

代码:

User.find().populate('roles').exec(function (err, data) {
 res.json(data);
})

在这里我可以在用户下获得角色，但是如何在每个角色下获得用户.

Here i can get roles under user, But how i get users under each role.