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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
}
Updated on: 30-Jul-2019
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