Find all duplicate documents in a MongoDB collection by a key field?

Use the aggregate framework to find all duplicate documents in a MongoDB collection by a key field.

To understand the concept, let us create a collection with the document. The query to create a collection with a document is as follows −

> db.findDuplicateByKeyDemo.insertOne({"StudentId":1,"StudentName":"John"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c7f5b168d10a061296a3c3a")
}
> db.findDuplicateByKeyDemo.insertOne({"StudentId":2,"StudentName":"Carol"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c7f5b1f8d10a061296a3c3b")
}
> db.findDuplicateByKeyDemo.insertOne({"StudentId":3,"StudentName":"Carol"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c7f5b248d10a061296a3c3c")
}
> db.findDuplicateByKeyDemo.insertOne({"StudentId":4,"StudentName":"John"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c7f5b2d8d10a061296a3c3d")
}
> db.findDuplicateByKeyDemo.insertOne({"StudentId":5,"StudentName":"Sam"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c7f5b398d10a061296a3c3e")
}
> db.findDuplicateByKeyDemo.insertOne({"StudentId":6,"StudentName":"Carol"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c7f5b438d10a061296a3c3f")
}

Display all documents from a collection with the help of find() method. The query is as follows −

> db.findDuplicateByKeyDemo.find().pretty();

The following is the output −

{
   "_id" : ObjectId("5c7f5b168d10a061296a3c3a"),
   "StudentId" : 1,
   "StudentName" : "John"
}
{
   "_id" : ObjectId("5c7f5b1f8d10a061296a3c3b"),
   "StudentId" : 2,
   "StudentName" : "Carol"
}
{
   "_id" : ObjectId("5c7f5b248d10a061296a3c3c"),
   "StudentId" : 3,
   "StudentName" : "Carol"
}
{
   "_id" : ObjectId("5c7f5b2d8d10a061296a3c3d"),
   "StudentId" : 4,
   "StudentName" : "John"
}
{
   "_id" : ObjectId("5c7f5b398d10a061296a3c3e"),
   "StudentId" : 5,
   "StudentName" : "Sam"
}
{
   "_id" : ObjectId("5c7f5b438d10a061296a3c3f"),
   "StudentId" : 6,
   "StudentName" : "Carol"
}

Here is the query to find all duplicate documents” −

> db.findDuplicateByKeyDemo.aggregate([
   ... { $group: {
      ... _id: { StudentName: "$StudentName" },
      ... UIDS: { $addToSet: "$_id" },
      ... COUNTER: { $sum: 1 }
   ... } },
   ... { $match: {
      ... COUNTER: { $gte: 2 }
   ... } },
   ... { $sort : { COUNTER : -1} },
   ... { $limit : 10 }
... ]).pretty();

The following is the output displaying the duplicate records. Here, the student „Carol‟ comes 3 times, whereas John 2 times −

The following is the output −

{
   "_id" : {
      "StudentName" : "Carol"
   },
   "UIDS" : [
      ObjectId("5c7f5b248d10a061296a3c3c"),
      ObjectId("5c7f5b438d10a061296a3c3f"),
      ObjectId("5c7f5b1f8d10a061296a3c3b")
   ],
   "COUNTER" : 3
}
{
   "_id" : {
      "StudentName" : "John"
   },
   "UIDS" : [
      ObjectId("5c7f5b2d8d10a061296a3c3d"),
      ObjectId("5c7f5b168d10a061296a3c3a")
   ],
   "COUNTER" : 2
}