Update field in exact element array in MongoDB?

To update a specific element in a nested array in MongoDB, use the $ positional operator to match the parent array element, then target the nested array element by its index position using dot notation.

Syntax

db.collection.update(
    {"arrayName.field": "matchValue"},
    { $set: { "arrayName.$.nestedArray.INDEX": "newValue" } }
);

Sample Data

Let's create a document with nested arrays to demonstrate the update operation ?

db.updateExactField.insertOne({
    "ActorId": 1,
    "ActorDetails": [
        {
            "ActorName": "Johnny Depp",
            "MovieList": ["The Tourist", "Public Enemy"]
        },
        {
            "ActorName": "Chris Evans",
            "MovieList": ["Captain America", "Avengers"]
        }
    ]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("...")
}

Case 1: Update at Index 2 (3rd Position)

Add "Avengers:Infinity War" at index 2 in Chris Evans' MovieList ?

db.updateExactField.update(
    { "ActorDetails.ActorName": "Chris Evans" },
    { $set: { "ActorDetails.$.MovieList.2": "Avengers:Infinity War" } }
);

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

The updated MovieList for Chris Evans:

"ActorName": "Chris Evans",
"MovieList": [
    "Captain America",
    "Avengers",
    "Avengers:Infinity War"
]

Case 2: Update at Index 1 (2nd Position)

Replace "Avengers" with "Gifted" at index 1 ?

db.updateExactField.update(
    { "ActorDetails.ActorName": "Chris Evans" },
    { $set: { "ActorDetails.$.MovieList.1": "Gifted" } }
);

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

The final MovieList for Chris Evans:

"ActorName": "Chris Evans",
"MovieList": [
    "Captain America",
    "Gifted",
    "Avengers:Infinity War"
]

Johnny Depp's MovieList remains unchanged because the $ operator only targets the matched element (Chris Evans).

Key Points

• $ matches the first parent array element that satisfies the query condition.
• Use dot notation with the numeric index (.0, .1, .2) to target a specific position in the nested array.
• Setting an index beyond the current array length creates the element at that position (sparse array).
• The positional operator works only with the first matched array element in a single update operation.

Conclusion

Combine the $ positional operator with numeric index dot notation (arrayName.$.nestedArray.N) to update exact elements in nested arrays. The $ identifies the matched parent element, and the index targets the specific nested position for precise array manipulation.