Batch multiple queries in mongodb nodejs

Question

I need some help with the following requirement, how would you do this in one batch, if this even possible in mongodb with nodejs?

I got multiple array's of 1000 id's from another source via the API. I need to get a result for each id.

Basically it should look like this:

const bulk = db.collection('profiles').initializeUnorderedBulkOp();

arrayWith1000TagIds.forEach((id) => {
  bulk
    .find({ tags: { $elemMatch: { _id: new ObjectId(id) } } })
    .count();

Unfortunately count is not a function of bulk. Any other way of doing this in one batch instead of doing a query for every id? In the end this is going to be 10k of queries.

Thank you. I'm pretty new to mongodb.

The result should look like this:

{
    [tagId]: 3, (There are 3 profiles, which include this specific tag in their tag list)
    [tagId]: 2,
    [tagId]: 0,
    ...
}

Answer 1

That is more easily done with a single aggregation, something like this:

db.collection.aggregate([
  {$unwind: "$tags"},
  {$match: {"tags._id" : {$in: arrayWith1000TagIds}}},
  {$group: {"_id" : "$tags._id", "count" : {$sum: 1}}}
])

(I have taken the liberty of assuming that "tags" is an array of objects, as suggested by your use of $elemMatch in your question).