Sort by deep document field in MongoDb - arrays

Question

I have a collection called Visitor which has an array of chats and each array has a document called user.
I need to find some documents on this collection and sort them by if they have some specific user in their chats first.
The path for the user id is:
chats.user._id
where:
chats // array
user // document
_id // ObjectId
The below script does sort the documents correctly, however, it expands the chats array and multiplies the document for each chat in the array.
I only need the sorting, so can I sort and not use the unwind pipeline or make it somehow not multiply the documents?
db.getCollection('Visitor').aggregate([
{$unwind: "$chats"},
{ $match: {'event._id':ObjectId('5c942a3591deb389bfd92579'), 'chats.enabled': {$exists: true}}},
{
"$project": {
"_id": 1,
"chats.user._id": 1,
"weight": {
"$cond": [
{ "$eq": [ "$chats.user._id", ObjectId("5c942a3591deb389bfd92579") ] },
10,
0
]
}
}
},
{ "$sort": { "weight": -1 } },
])
EDIT: I don't need to sort the inner array, but sort the find command by checking if a specific user is in the chats array.
Some sample of Visitor collection:
[
{
"_id" : ObjectId("5c9a3a1bd86e0ba64106e90e"),
"event" : {
"_id" : ObjectId("5c942a3591deb389bfd92579")
},
"chats" : [
{
"enabled" : false,
"user" : {
"_id" : ObjectId("5c81232f09a923b559763418")
},
"_id" : ObjectId("5c9a3a1bd86e0ba64106e915")
}
]
},
{
"_id" : ObjectId("5c9a3a35d86e0ba64106e950"),
"event" : {
"_id" : ObjectId("5c942a3591deb389bfd92579")
},
"chats" : [
{
"enabled" : true,
"user" : {
"_id" : ObjectId("5c81232f09a923b559763418")
},
"_id" : ObjectId("5c9a3a35d86e0ba64106e957")
},
{
"enabled" : true,
"user" : {
"_id" : ObjectId("5c942a3591deb389bfd92579")
},
"_id" : ObjectId("5c9a3a34d86e0ba64106e91d")
}
]
}
]
In the above sample, I need to make the second document to be sorted first because it has the user with the _id ObjectId("5c942a3591deb389bfd92579").

Answer 1

The problem here is that using $unwind you modify initial structure of your documents (you will get one document per chats. I would suggest using $map to get an array of weights based on specified userId and then you can use $max to get final weight
db.col.aggregate([
{ $match: {'event._id':ObjectId('5c942a3591deb389bfd92579'), 'chats.enabled': {$exists: true}}},
{
"$project": {
"_id": 1,
"chats.user._id": 1,
"weight": {
$max: { $map: { input: "$chats", in: { $cond: [ { $eq: [ "$$this.user._id", ObjectId("5c942a3591deb389bfd92579") ] }, 10, 0 ] } } }
}
}
},
{ "$sort": { "weight": -1 } },
])