How to remove all documents from a collection except a single document in MongoDB?

To remove all documents from a collection except a single document in MongoDB, use remove() based on some condition. Let us create a collection with documents. Following is the query

>db.removeAllDocumentsExceptOneDemo.insertOne({"StudentName":"Larry","StudentAge":21});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c9c9de42d66697741252478")
}
>db.removeAllDocumentsExceptOneDemo.insertOne({"StudentName":"Mike","StudentAge":21,"StudentCountryName":"US"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c9c9dea2d66697741252479")
}
>db.removeAllDocumentsExceptOneDemo.insertOne({"StudentName":"Chris","StudentAge":24,"StudentCountryName":"AUS"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c9c9def2d6669774125247a")
}

Following is the query to display all documents from a collection with the help of find() method

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

This will produce the following output

{
   "_id" : ObjectId("5c9c9de42d66697741252478"),
   "StudentName" : "Larry",
   "StudentAge" : 21
}
{
   "_id" : ObjectId("5c9c9dea2d66697741252479"),
   "StudentName" : "Mike",
   "StudentAge" : 21,
   "StudentCountryName" : "US"
}
{
   "_id" : ObjectId("5c9c9def2d6669774125247a"),
   "StudentName" : "Chris",
   "StudentAge" : 24,
   "StudentCountryName" : "AUS"
}

Following is the query to remove all documents from a collection except a single document i.e. with StudentAge 24

> db.removeAllDocumentsExceptOneDemo.remove({ StudentAge: { $ne: 24 } } );
WriteResult({ "nRemoved" : 2 })

Let us check all the documents now. Following is the query

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

Following is the output displaying only a single document

{
   "_id" : ObjectId("5c9c9def2d6669774125247a"),
   "StudentName" : "Chris",
   "StudentAge" : 24,
   "StudentCountryName" : "AUS"
}