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 }
]
}