Unwind inside lookup in Mongoose - node.js

Question

I have 2 entities to merge: Customer and Feedback. Feedback contains an embedded array of upvotes (Upvote)
A customer is not able to upvote more than once for a specific feedback.
What I would like to achieve is - given a specific feedback id - get the complete list of customers with an additional virtual attribute that states whether he/she upvoted the given feedback.
Customer.aggregate(
[
{
$match: { company_id: new ObjectID(req.user.company_id) }
},
{
$lookup: {
from: 'feedbacks',
let: { 'c_id': '$_id' },
pipeline: [
{
$unwind: '$upvotes'
},
{
$match: { $expr: { $eq: ['$upvotes.customer_id._id', '$$c_id'] } }
}
],
as: 'upvotes'
}
}
],
function(err, customers) {
if (err) {
console.log(err);
res.status(400).send(err);
} else {
res.send({ customers });
}
}
);
To do that I have to look through the list of upvotes for that specific feedback and, then, join it with the customer table using the customer_id.
The above mentioned approach does not work. Any suggestion what I am doing wrong?
Sample data (Feedback)
{
"size": 0,
"points": 50,
"status": "open",
"potential": 0,
"real": 5000,
"_id": "5c3d033271ceb7edc37d156c",
"title": "Custom Invoice Templates",
"description": "Provide an editor to create custom invoices.",
"owner_id": {
"_id": "5c3b684f7cec8be977c2a465",
"email": "maurizio#acme.com"
},
"company_id": "5c3b684f7cec8be977c2a462",
"project_id": "5c3b68507cec8be977c2a468",
"upvotes": [
{
"_id": "5c3fa5b371ceb7edc37d159a",
"comments": "bbbb",
"priority": "should",
"customer_id": {
"size": 0,
"potential": 0,
"real": 5000,
"_id": "5c3b68507cec8be977c2a485",
"name": "Oyomia Ltd."
},
"owner_id": {
"_id": "5c3b684f7cec8be977c2a465",
"email": "maurizio#acme.com"
}
}
],
"updatedAt": "2019-01-16T21:44:19.215Z",
"createdAt": "2019-01-14T21:46:26.286Z",
"__v": 0
}
Sample data (Customer)
{
"size": 0,
"potential": 0,
"real": 5000,
"_id": "5c3b68507cec8be977c2a485",
"name": "Oyomia Ltd.",
"contact": {
"_id": "5c40f8de71ceb7edc37d15ab",
"name": "Nick Page",
"email": "np#oyoma.com"
},
"company_id": "5c3b684f7cec8be977c2a462",
"deals": [
{
"value": 5000,
"_id": "5c3b68507cec8be977c2a487",
"name": "Armour batch",
"status": "won",
"type": "non_recurring",
"updatedAt": "2019-01-13T16:33:20.870Z"
}
],
"__v": 0,
"updatedAt": "2019-01-17T21:51:26.877Z"
}