Styjun

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").

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 ,
])