How to delete n-th element of array in MongoDB?

You can use $unset as well as $pull operator with an update to delete the nth element of an array.

Let us create a collection with a document. The query to create a collection with a document is as follows −

> db.getNThElementDemo.insertOne({"UserName":"John","UserAge":23,"ListOfFriends":["Carol","Sam","Mike","Bob"]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c94ee7516f542d757e2b43e")
}
> db.getNThElementDemo.insertOne({"UserName":"David","UserAge":21,"ListOfFriends":["Chris","Robert"]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c94eeaa16f542d757e2b43f")
}

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

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

The following is the output −

{
   "_id" : ObjectId("5c94ee7516f542d757e2b43e"),
   "UserName" : "John",
   "UserAge" : 23,
   "ListOfFriends" : [
      "Carol",
      "Sam",
      "Mike",
      "Bob"
   ]
}
{
   "_id" : ObjectId("5c94eeaa16f542d757e2b43f"),
   "UserName" : "David",
   "UserAge" : 21,
   "ListOfFriends" : [
      "Chris",
      "Robert"
   ]
}

Here is the query to delete the nth element of an array −

> db.getNThElementDemo.update({}, {$unset : {"ListOfFriends.2" : 1 }});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

> db.getNThElementDemo.update({}, {$pull : {"ListOfFriends" : null}});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Now you can check nth element from an array has been removed.

The query is as follows −

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

The following is the output −

{
   "_id" : ObjectId("5c94ee7516f542d757e2b43e"),
   "UserName" : "John",
   "UserAge" : 23,
   "ListOfFriends" : [
      "Carol",
      "Sam",
      "Bob"
   ]
}
{
   "_id" : ObjectId("5c94eeaa16f542d757e2b43f"),
   "UserName" : "David",
   "UserAge" : 21,
   "ListOfFriends" : [
      "Chris",
      "Robert"
   ]
}