mongodb - check a collection for mutual likes - mongodb

Question

I had a query that worked before when likes collection had users stored as from and to fields. The query below checked for the two fields and if like was true.
Since then I changed the way users are stored in the likes collection.
.collection('likes')
.aggregate([ {$match: {$and : [{$or: [{from: userId}, {to: userId}]}, {like: true}]}},
{$group: {
_id: 0,
from: {$addToSet: "$from"},
to: {$addToSet: "$to"},
}
},
{$project: {
_id: 0,
users: {
$filter: {
input: {$setIntersection: ["$from", "$to"]},
cond: {$ne: ["$$this", userId]}
}
}
}
},
This is how likes collection used to store data (and above query worked) in figuring out mutual likers for a userId passed in req.body
{
"_id": "xw5vk1s_PpJaal46di",
"from": "xw5vk1s",
"to": "PpJaal46di"
"like": true,
}
and now I changed users to an array.
{
"_id": "xw5vk1s_PpJaal46di",
"users": [
"xw5vk1s",//first element equivalent to from
"PpJaal46di"//equivalent to previously to
],
"like": true,
}
I am not sure how to modify the query to now check for array elements in users field now that from and to is not where the two users liking each other are stored.

Answer 1

For MongoDB v5.2+, you can use $sortArray to create a common key for the field users and $group to get the count. You will get mutual likes by count > 1.
db.collection.aggregate([
{
$match: {
users: "xw5vk1s"
}
},
{
$group: {
_id: {
$sortArray: {
input: "$users",
sortBy: 1
}
},
count: {
$sum: 1
}
}
},
{
"$match": {
count: {
$gt: 1
}
}
}
])
Mongo Playground
Prior to MongoDB v5.2, you can create sorted key for grouping through $unwind and $sort then $push back the users in $group.
db.collection.aggregate([
{
$match: {
users: "xw5vk1s"
}
},
{
"$unwind": "$users"
},
{
$sort: {
users: 1
}
},
{
$group: {
_id: "$_id",
users: {
$push: "$users"
},
like: {
$first: "$like"
}
}
},
{
"$group": {
"_id": "$users",
"count": {
"$sum": 1
}
}
},
{
"$match": {
count: {
$gt: 1
}
}
}
])
Mongo Playground