Implement $dateToString on array items with MongoDB

To implement $dateToString on array items, use aggregate() in MongoDB. Let us create a collection with documents −

> db.demo104.insertOne(
...    {
...
...       "AppName" : "Online Book",
...       "Details" : [
...          {
...             "ClientName" : "Chris",
...             "Deadline" : new ISODate("2020-03-10")
...          }
...       ]
...    }
... );
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e2ed7fd9fd5fd66da21446f")
}

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

> db.demo104.find().pretty();

This will produce the following output −

{
   "_id" : ObjectId("5e2ed7fd9fd5fd66da21446f"),
   "AppName" : "Online Book",
   "Details" : [
      {
         "ClientName" : "Chris",
         "Deadline" : ISODate("2020-03-10T00:00:00Z")
      }
   ]
}

Following is the query to implement $dateToString on array items −

> db.demo104.aggregate([
...    { "$match": {}},
...       { "$project": {
...          "title": 1,
...          "Details": {
...             "$map": {
...                "input": "$Details",
...                "as": "out",
...                "in": {
...                   "ClientName": "$$out.ClientName",
...                   "Deadline": {
...                   "$dateToString": { "format": "%m", "date": "$$out.Deadline" }
...                }
...             }
...          }
...       }
...    }}
... ])

This will produce the following output −

{ "_id" : ObjectId("5e2ed7fd9fd5fd66da21446f"), "Details" : [ { "ClientName" : "Chris", "Deadline" : "03" } ] }