How to insert an item to an array that is inside an object in MongoDB?

To insert an item to an already created array inside an object, use MongoDB $push. Let us create a collection with documents −

> db.demo449.insertOne(
... {
...    details1: {
...       details2: [{
...          _id:new ObjectId(),
...             Name:"Chris"
...       }],
...       details3: [{
...          _id:new ObjectId(),
...          Name:"David"
...       }]
...    }
... }
... );
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e7a40e971f552a0ebb0a6e3")
}

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

> db.demo449.find();

This will produce the following output −

{ "_id" : ObjectId("5e7a40e971f552a0ebb0a6e3"), "details1" : { "details2" : [ { "_id" :
ObjectId("5e7a40e971f552a0ebb0a6e1"), "Name" : "Chris" } ], "details3" : [ { "_id" :
ObjectId("5e7a40e971f552a0ebb0a6e2"), "Name" : "David" } ] } }

Following is the query to insert an item to an array that is inside an object −

> db.demo449.update({_id:ObjectId("5e7a40e971f552a0ebb0a6e3")}, {$push: {
'details1.details2':{_id:ObjectId(),"Name":"Carol"}}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 }

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

> db.demo449.find();

This will produce the following output −

{ "_id" : ObjectId("5e7a40e971f552a0ebb0a6e3"), "details1" : { "details2" : [ { "_id" :
ObjectId("5e7a40e971f552a0ebb0a6e1"), "Name" : "Chris" }, { "_id" :
ObjectId("5e7a41a671f552a0ebb0a6e5"), "Name" : "Carol" } ], "details3" : [ { "_id" :
ObjectId("5e7a40e971f552a0ebb0a6e2"), "Name" : "David" } ] } }