mongoose populate two simple schemas - node.js

Question

I am stuck with mongoose populate() method.
Here is my schemas:
const GroupSchema = Schema({
title: String,
ips: [
{
ip: String,
hostname: String,
added : { type : Date, default: Date.now }
}
]
});
module.exports = mongoose.model('groups', GroupSchema);
const UserSchema = Schema({
username: String,
ip: String,
groups: [
{
_id : { type: Schema.Types.ObjectId, ref: 'groups'},
added : { type : Date, default: Date.now }
}
]
});
module.exports = mongoose.model('users', UserSchema);
I am trying to join users.groups[]._id with groups._id but absolutely no luck.
Here is how I tried:
User.find().
populate('groups').
exec(function (err, user) {
if (err) return handleError(err);
console.log(user);
});
getting this result:
{ _id: 5b3e039a2f714d38ccf66cax,
username: 'rrrr',
ip: '10.1.1.1',
groups: [ [Object], [Object] ],
__v: 0 } ]
I want to get like this:
{ _id: 5b3e039a2f714d38ccf66cax,
username: 'rrrr',
ip: '10.1.1.1',
groups: [{ title: sssss, added: ssss }, {title: xxxx, added: ssss}] ],
__v: 0 } ]

Answer 1

You can try using $lookup aggregation
If you are using mongodb version 3.4 and below
User.aggregate([
{ "$unwind": "$groups" },
{ "$lookup": {
"from": Groups.collection.name,
"let": { "groupId": "$groups._id" },
"pipeline": [
{ "$match": { "$expr": { "$eq": [ "$_id", "$$groupId" ] } } }
],
"as": "groups._id"
}},
{ "$unwind": "$groups._id" },
{ "$group": {
"_id": "$_id",
"username": { "$first": "$username" },
"ip": { "$first": "$ip" },
"groups": { "$push": "$groups" }
}}
])
If you are using mongodb version 3.6 and above
User.aggregate([
{ "$unwind": "$groups" },
{ "$lookup": {
"from": Groups.collection.name,
"localField": "groups._id",
"foreignField": "_id",
"as": "groups._id"
}},
{ "$unwind": "$groups._id" },
{ "$group": {
"_id": "$_id",
"username": { "$first": "$username" },
"ip": { "$first": "$ip" },
"groups": { "$push": "$groups" }
}}
])