Mongodb aggregate match and group documents, with additional check - mongodb

Question

So I have 2 models: Question and Answer.
Answer has: questionId, userId, answer (String).
I need an aggregation pipline that will:
match all answers by questionId
see if the current user already voted (is his id in matched documents)
group answers and count them
I implemented 1 and 3 like this:
const q = ObjectId('5d6e52a68558b63fb9302efd');
const user = ObjectId('5d0b3f7daceeb50c477b49e0');
Answer.aggregate([
{ $match: { questionId: q } },
{ $group: { _id: '$answer', count: { $sum: 1 } } },
])
I am missing a step between those 2 aggregation pipelines, where I would iterate thru matched documents, and check if userId matches user.
I would like to get some object like this:
{
didIVote: true,
result: [ { _id: 'YES', count: 5 }, { _id: 'NO', count: 2 } ]
}
Or maybe even like this:
[
{ _id: 'YES', count: 5, didIVote: true },
{ _id: 'NO', count: 2, didIVote: false },
]

Answer 1

In the $group stage, create an array with the users that voted
for each answer.
Add an aditional $project stage to check if the user is in the array.
const q = ObjectId('5d6e52a68558b63fb9302efd');
const user = ObjectId('5d0b3f7daceeb50c477b49e0');
Answer.aggregate([
{ $match: { questionId: q } },
{
$group: {
_id: '$answer',
count: { $sum: 1 },
voted: { $addToSet: "$userId" }
}
},
{
$project: {
count: 1,
didIVote: { $in: [ user, "$voted" ] },
}
}
]);