How to add a sub-document to sub-document array in MongoDB?

To add a sub-document to a sub-document array in MongoDB, use the $push operator. This operator appends a new document to an existing array field within your MongoDB collection.

Syntax

db.collection.update(
    { "matchField": "value" },
    { "$push": { "arrayField": { newSubDocument } } }
)

Sample Data

Let us first create a collection with documents ?

db.subDocumentToSubDocumentDemo.insertOne(
    {
        "_id": 101,
        "StudentName": "Larry",
        "StudentAge": 21,
        "StudentDetails": [
            {
                "StudentCountryName": "US",
                "StudentFavouriteSubjectList": [ ]
            }
        ]
    }
)

{ "acknowledged": true, "insertedId": 101 }

Following is the query to display all documents from a collection with the help of find() method ?

db.subDocumentToSubDocumentDemo.find().pretty()

{
    "_id": 101,
    "StudentName": "Larry",
    "StudentAge": 21,
    "StudentDetails": [
        {
            "StudentCountryName": "US",
            "StudentFavouriteSubjectList": [ ]
        }
    ]
}

Example: Adding Sub-Document to Array

Following is the query to add a sub-document to sub-document array in MongoDB ?

db.subDocumentToSubDocumentDemo.update(
    { "_id": 101 },
    {
        "$push": {
            "StudentDetails": {
                "StudentCountryName": "UK",
                "StudentFavouriteSubjectList": ["MongoDB", "Java"]
            }
        }
    }
)

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1 })

Verify Result

Let us check all the documents once again ?

db.subDocumentToSubDocumentDemo.find().pretty()

{
    "_id": 101,
    "StudentName": "Larry",
    "StudentAge": 21,
    "StudentDetails": [
        {
            "StudentCountryName": "US",
            "StudentFavouriteSubjectList": [ ]
        },
        {
            "StudentCountryName": "UK",
            "StudentFavouriteSubjectList": [
                "MongoDB",
                "Java"
            ]
        }
    ]
}

Conclusion

The $push operator successfully adds new sub-documents to existing arrays in MongoDB. This operation appends the new document to the end of the specified array field without affecting existing elements.