MongoDB - Matching the two collections by field name

Question

I have two collections.

collection1:

[
  {
    "_id": "user1"
  },
  {
    "_id": "user2"
  }
]

collection2:

[
  {
    "name": "test",
    "Info": {
      "initiatedBy": "user1"
    }
  },
  {
    "name": "Test LFE",
    "Info": {
      "initiatedBy": "user1"
    }
  },
  {
    "name": "Test uuE",
    "Info": {
      "initiatedBy": "user2"
    }
  }
]

I want the final result as

user1: 2. Because user1 created 2 records.

user2: 1. Because only one record was created by user2.

I used the below query to execute in Mongo Compass but was not given the exact result.

db.collection1.aggregate([
  {
    $lookup: {
      from: "collection2",
      localField: "_id",
      foreignField: "Info.initiatedBy",
      as: "matchedRecords"
    }
  },
  {
    $unwind: "$matchedRecords"
  },
  {
    $group: {
      _id: null,
      count: {
        $sum: "$matchedRecords.fieldToCount"
      }
    }
  }
])

Please provide your suggestions to correct the above query.

Answer 1

I don't see that any reason to $unwind the matchedRecords array and perform $group.

Instead, you can get the size of matchedRecords array with $size operator.

db.collection1.aggregate([
  {
    $lookup: {
      from: "collection2",
      localField: "_id",
      foreignField: "Info.initiatedBy",
      as: "matchedRecords"
    }
  },
  {
    $project: {
      _id: 1,
      count: {
        $size: "$matchedRecords"
      }
    }
  }
])

Demo @ Mongo Playground