MongoDB $push in nested array?

The $push operator adds elements to an array field. To push into a nested array (an array inside another array), use the $ positional operator to identify the matched parent array element, then target the nested array with dot notation.

Sample Data

db.nestedArrayDemo.insertOne({
    "EmployeeName": "Larry",
    "EmployeeSalary": 9000,
    "EmployeeDetails": [{
        "EmployeeDOB": new Date("1990-01-21"),
        "EmployeeDepartment": "ComputerScience",
        "EmployeeProject": [
            {"Technology": "C", "Duration": 6},
            {"Technology": "Java", "Duration": 7}
        ]
    }]
});

Push into Nested Array

Add a new project to the EmployeeProject array inside EmployeeDetails ?

db.nestedArrayDemo.update(
    {
        "_id": ObjectId("5c6d73090c3d5054b766a76e"),
        "EmployeeDetails.EmployeeDepartment": "ComputerScience"
    },
    {
        $push: {
            "EmployeeDetails.$.EmployeeProject": {
                "Technology": "Python",
                "Duration": 4
            }
        }
    }
);

The $ in EmployeeDetails.$ refers to the first element in EmployeeDetails that matches EmployeeDepartment: "ComputerScience". Then .EmployeeProject targets the nested array within that matched element.

Verify Result

db.nestedArrayDemo.find().pretty();

{
    "EmployeeName": "Larry",
    "EmployeeSalary": 9000,
    "EmployeeDetails": [{
        "EmployeeDOB": ISODate("1990-01-21T00:00:00Z"),
        "EmployeeDepartment": "ComputerScience",
        "EmployeeProject": [
            {"Technology": "C", "Duration": 6},
            {"Technology": "Java", "Duration": 7},
            {"Technology": "Python", "Duration": 4}
        ]
    }]
}

Python with Duration 4 has been successfully added to the nested EmployeeProject array.

Conclusion

To $push into a nested array, match the parent array element in the query filter, then use $ (positional operator) + dot notation to target the nested array. For deeper nesting or multiple matches, use $[<identifier>] (arrayFilters) in MongoDB 3.6+.