Find duplicate name in MongoDB

March 8, 2022

I’m having a problem in getting the duplicate name in my mongodb to delete duplicates.

{
    "users": [
      {
        "_id": {
          "$oid": "61441890a6566a001623b8ed"
        },
        "name": "Jollibee",
      },
      {
        "_id": {
          "$oid": "61441890a6566a001623b8ed"
        },
        "name": "Jollibee",
      },
      {
        "_id": {
          "$oid": "61441890a6566a001623b8ed"
        },
        "name": "MCDO",
      },
      {
        "_id": {
          "$oid": "61441890a6566a001623b8ed"
        },
        "name": "Burger King",
      },
    ]
  }

I want to show in my output only the duplicate names. which is Jollibee.

tried this approach but it only returns me the count of all the users not the duplicated ones. I want to show 2 Jollibee only.

db.collection.aggregate([
  {
    "$unwind": "$users"
  },
  {
    "$group": {
      "_id": "$_id",
      "count": {
        "$sum": 1
      }
    }
  },
  {
    "$match": {
      "_id": {
        "$ne": null
      },
      "count": {
        "$gt": 1
      }
    }
  }
])

>Solution :

since the $unwind step gives you same _id for all documents grouping by _id is not correct. Instead try grouping by users.name

db.collection.aggregate([
  {
    "$unwind": "$users"
  },
  {
    "$group": {
      "_id": "$users.name",
      "count": {
        "$sum": 1
      }
    }
  },
  {
    "$match": {
      "_id": {
        "$ne": null
      },
      "count": {
        "$gt": 1
      }
    }
  }
])

demo