How to update Reference Array in mongoose - node.js

Question

I have a Group Collection, which is having a Reference array of Members. Two Objects are inter-connected like follow. When I am adding new members to the group the members field of Group object needed to be updated. How can I do this with a mongoose update operator.
var MemberSchema = new Schema({
name:{
type:String,
default:null
},
user_id:{
type : Schema.ObjectId,
ref : 'User',
default : null
}
});
var GroupSchema = new Schema({
name:{
type:String,
default:null
},
description:{
type:String,
default:null
},
members:[MemberSchema],
},{collection:"groups"});
Thank You in advance.
Update
I added a sample document of group.
{
"_id" : ObjectId("586a2e694467c41218b302c3"),
"members" : [
{
"_id" : ObjectId("586a2e694467c41218b302c6"),
"user_id" : ObjectId("58171d75e72bf516f92dcd4e"),
"name" : "Lakmal Kapukotuwa"
},
{
"_id" : ObjectId("586a2e694467c41218b302c5"),
"user_id" : ObjectId("5821807516325e127f59438e"),
"name" : "Prasad Perera"
},
{
"_id" : ObjectId("586a2e694467c41218b302c4"),
"user_id" : ObjectId("586263515356e908de6c899a"),
"name" : "Sadun Prasad"
}
],
"description" : "Des 1",
"name" : "My group",
"__v" : 0
}

Answer 1

If you are sending the new members as a list of objects with the following structure e.g.
membersListToAdd = [
{
"user_id": "58171d75e72bf516f92dcd4e",
"name": "foo"
},
{
"user_id": "5821807516325e127f59438e",
"name": "bar"
}
]
then use $push with $each modifier in an update as follows:
var query = { name: 'My Group' },
options = {},
callback = function (err, result) { console.log(result); };
Group.update(query, { $push: { members: { $each: membersListToAdd } } }, options, callback)

Answer 2

You are doing this wrong,
no need to have links in both collections and no need to nest models
try this instead
var Group = mongoose.model("Group", new Schema({
name: {
type:String
},
description: {
type:String
},
}));
Group.virtual("users", {
ref: "User",
localField: "_id",
foreignField: "groups"
});
var User = mongoose.model("User", new Schema({
name:{
type:String
},
groups: [{
type : Schema.ObjectId,
ref : 'Group'
}]
}));