MongoDB query on nth element (variable index) of subdocument array

For this, use $let along with $expr. Here, $let is used to define temporary variable. The $expr is used for aggregation expressions. Let us create a collection with documents −

> db.demo72.insertOne(
... {
...    StudentDetails:[
...       {
...          Name: "Chris",
...          Age: 21
...       },
...       {
...          Name: "David",
...          Age: 23
...       },
...       {
...          Name: "Bob",
...          Age: 20
...       },
...    ]
... }
... );
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e29b4ad71bf0181ecc42269")
}

Display all documents from a collection with the help of find() method −

> db.demo72.find().pretty();

This will produce the following output −

{
   "_id" : ObjectId("5e29b4ad71bf0181ecc42269"),
   "StudentDetails" : [
      {
         "Name" : "Chris",
         "Age" : 21
      },
      {
         "Name" : "David",
         "Age" : 23
      },
      {
         "Name" : "Bob",
         "Age" : 20
      }
   ]
}

Following is the query on nth element (variable index) of subdocument array −

> db.demo72.find({
... $expr: {
...    $let: {
...       vars: { fst: { $arrayElemAt: [ "$StudentDetails", 1 ] } },
...       in: { $eq: [ "$$fst.Name", "David" ] }
...       }
...    }
... })

This will produce the following output −

{
   "_id" : ObjectId("5e29b4ad71bf0181ecc42269"), "StudentDetails" : [
      { "Name" : "Chris", "Age" : 21 }, { "Name" : "David", "Age" : 23 }, { "Name" : "Bob", "Age" : 20 }
   ]
}