Mongoose aggregate - $filter not removing matching elements

Question

I have the following query that should update the privacyStatus of a user to some value, and also remove all notifications (an array of ObjectIds) which have a type property equal to the value: follow_request.

 User.aggregate([
            {
                $match: { _id: new mongoose.Types.ObjectId(req.user._id) },
            },
            {
                $lookup: {
                    from: "Notification",
                    localField: "notifications",
                    foreignField: "_id",
                    as: "notifications",
                },
            },
            {
                $set: {
                    privacyStatus: value,
                    notifications: {
                        $filter: {
                            input: "$notifications",
                            cond: {
                                $ne: ["$$this.type", "follow_request"],
                            },
                        },
                    },
                },
            },
    
            {
                $merge: {
                    into: "users",
                },
            },
        ]).then((result) => {
            res.send("Done")
        })

The privacyStatus is successfully updated in the database, therefore I know the $match stage is working correctly, however the notifications array doesn't update at all even if there are notifications that match the filter.

My User model:

{
  privacyStatus: {type: Boolean},
  notifications: [type: mongoose.Schema.Types.ObjectId,ref: "Notification",]
}

Notification Model:

{
   type: {type: String, default: "follow_request"}
}

Why aren't my notifications that have a type of follow_request being removed from the users notification array? Thanks

Sample documents: user document:

{
   _id: ObjectId('66324110da2f0f7175ca1949'),
   username: "sam",
   notifications: [
      ObjectId('663c1c0bd0sa68fae7cf6ad2')
   ]
}

Notification document:

{
   _id: ObjectId('663c1c0bd0sa68fae7cf6ad2'),
   type: "follow_request"
}

Answer 1

To get your aggregate to merge the results back intot he users collection you need to do two things:

1. Change the collection used in the $lookup from Notification to notifications.
2. Recognise that your User.notifications field is an array of ObjectIds. When your $filter returns it's results, you won't be able $merge them back into your User.notifications array because they will be full Notification documents. That can be fixed with a $map stage to step over each filtered Notification document and only return their _id. Then that will match your schema.

The finished aggregation might look like this:

User.aggregate([
  {
    $match: {
       _id: new mongoose.Types.ObjectId(req.user._id)
    }
  },
  {
    $lookup: {
      from: "notifications", //< change to this
      localField: "notifications",
      foreignField: "_id",
      as: "notifications"
    }
  },
  {
    $set: {
      privacyStatus: value,
      notifications: {
        $map: {
          input: {
            $filter: {
              input: "$notifications",
              cond: {
                $ne: [
                  "$$this.type",
                  "follow_request"
                ]
              }
            }
          },
          as: "n",
          in: "$$n._id"
        }
      }
    }
  },
  {
    $merge: {
      into: "users"
    }
  }
]).then((result) => {
   res.send("Done")
})

See HERE for a working example.