MongoDB Aggregation Query: Find Documents Having Count More Than 1

In MongoDB, you can use aggregation queries to perform complex data processing operations. One common task is to find documents that have a count greater than 1.

Since MongoDB version 5.0, one option is to use $setWindowFields:

db.collection.aggregate([
  {$setWindowFields: {
      "partitionBy": "$uniqueKey",
      "sortBy": {"cust_id": 1},
      "output": {
        "count": {
          "$sum": 1,
          "window": {documents: ["unbounded", "unbounded"]}
          }
        }
      }
  }},
  {$match: {count: {$gt: 1}}},
  {$project: {_id: 0, "uniqueKey": 1, "cust_name": 1}}
])

See how it works on the playground example

This will prevent pushing all the documents with the same uniqueKey into one big document.

Before mongoDB 5.0:

One option is to use $group, but you need to use $push on your $group and $unwind afterwards. This option may be problematic if you have many documents with the same uniqueKey