MongoDB - get documents with the size of a nested array after filtering

Question

Problem:

I'm trying to get a list of documents and for each one of them to calculate the number of occurrences of a given value in a nested array of the same document.

I have a working example using the aggregation framework, but I wonder if there is a better way to accomplish the same thing, so I can benchmark the different approaches.

Simplified Data Model:

Single document in collection "Raffles":

{
  "_id" : objectId,
  "name" : string,
  "ends_at" : ISODate/Timestamp,
   ...
  "subscribers" : string[] //List of user ids
}

• The collections consists of documents representing a raffle/sweepstake with a name and start/ending dates.
• Users can subscribe to a raffle.
• Users can subscribe multiple times for the same raffle.

95% of the read queries will require both the raffle data like name, description and dates + information about the subscribed users. That's why I decided to have everything in a single raffle document instead of: referencing the subscribed raffles in the user document or having a separate collection with raffles and subscription counts.

Alternative maybe?:

The subscribers array is a list of strings representing the user's ID. This way adding a subscriber is as simple as pushing a new value. The other option is to have an array of objects like this and incrementing the count:

{
  "subscribers: [
     {
       "id": objectId  //User id
       "count": integer //Number of subscriptions
     },
     ...
  ]
}

Expected result:

The expected result is to have the full raffle document + an additional value of how many subscriptions a given user has.

{
    "_id" : objectId,
    "name" : string,
    "ends_at" : ISODate/Timestamp,
     ...
    "subscriptions" : 3 //Number of entries for a given user
}

Current solution

I'm getting the size after filtering the nested array with a given user id < USER_ID >

db.raffles.aggregate([
    ...
    {
        $project: {
            "name" : 1,
            "ends_at" :1,
             ...
            "subscriptions" : {
                 $size : {
                    $filter : {
                        input: "$subscribers",
                        as: "user",
                        cond: {
                            $eq: ["$$user", <USER_ID>]
                        },
                    }
                 }
            }
        }
    }
    ...
])

Questions:

• Are there other/better ways to accomplish the result from the current solution? Maybe grouping and summing up or map/reduce?

• Is it worth keeping not only the user ids, but rather objects with user id and subscription count?

• The current solution will throw an error if the subscriptions array is not set. Is there a way to handle this?

Thank you very much for the time spent reading this long post!

Answer 1

I would keep both user id and count in the subscription array and increment the count when you have a match with user id.

Something like

db.Raffles.update({"subscriptions.id":userid}, {$inc:{"subscriptions.$.count":1}}})

You can access the code using below query.

db.Raffles.find({"subscriptions.id":userid},{"name":1,"ends_at":1,"subscriptions.$":1});