node mongoose : aggregate not giving expected result - node.js

Question

withe the following Group Schema,
group.model.js
const Role = new mongoose.Schema({
name: { type: String, required: true }, // ensure uniqueness withn group instance using addToSet
description: { type: String, required: false }
});
const GroupSchema = new mongoose.Schema({
name: { type: String, index: { unique: true, required: true, dropDups: true } },
description: { type: String, required: false },
roles: [Role],
createdAt: {
type: Date,
default: Date.now
}
});
I am trying to list all roles ( subdocument) got a specific group
group.controller.js
function listRoles(req, res) {
const group = req.group;
console.log('GROUP: %j', group);
const limit = parseInt(req.query.limit, 10) || 50;
const skip = parseInt(req.query.skip, 10) || 0;
Group.aggregate([
{ $match: { _id: req.params.groupId } },
{ $unwind: '$roles' },
{ $skip: skip },
{ $limit: limit }
], (err, result) => {
if (err) {
res.status(500);
res.json({ message: 'Error. Cannot list roles', errror: err });
}
res.status(200);
console.log('RESULT: %j', result);
res.json(result);
});
}
I should get an array with one role, but I get an empty array
what's wrong with my aggregate code ? thanks for feedback
note: I tried to aggregate only with the $match in the pipe and I also get an empty array... so I guess. the issue comes from the req.params.groupId should be an ObjectId .. how can I cast it ?
console.log
GROUP: {"_id":"5923e2e83afd4149bdf16c61","name":"Admin","description":"Administration group","__v":1,"createdAt":"2017-05-23T07:21:12.470Z","roles":[{"name":"Role1","description":"description role1","_id":"5923e2e83afd4149bdf16c62"}]}
RESULT: []

Answer 1

To better diagnose this, I'd recommend removing steps from your aggregation pipeline and seeing what the result is. However, I suspect your problem is because you have no match at the first stage because you're comparing a string to an ObjectId. Try this:
const mongoose = require('mongoose')
// and in the aggregation:
{ $match: { _id: mongoose.Types.ObjectId(req.params.groupId) } }