How to select a specific subdocument in MongoDB?

To select a specific subdocument in MongoDB, use the find() method with dot notation to query array elements and the $ positional operator in projection to return only the matched subdocument.

Syntax

db.collection.find(
    { "arrayField.subfield": "value" },
    { "arrayField.$": 1, "_id": 0 }
);

Sample Data

Let us create a collection with documents containing subdocument arrays ?

db.demo37.insertMany([
    {
        "Details": [
            { "Name": "Chris", "Age": 21 },
            { "Name": "David", "Age": 23 }
        ]
    },
    {
        "Details": [
            { "Name": "Sam", "Age": 23 },
            { "Name": "Robert", "Age": 25 }
        ]
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e176635cfb11e5c34d898d7"),
        ObjectId("5e17664acfb11e5c34d898d8")
    ]
}

Display All Documents

db.demo37.find();

{ "_id": ObjectId("5e176635cfb11e5c34d898d7"), "Details": [ { "Name": "Chris", "Age": 21 }, { "Name": "David", "Age": 23 } ] }
{ "_id": ObjectId("5e17664acfb11e5c34d898d8"), "Details": [ { "Name": "Sam", "Age": 23 }, { "Name": "Robert", "Age": 25 } ] }

Example: Select Specific Subdocument

Select the subdocument where Name is "Sam" using the positional operator ?

db.demo37.find(
    { "Details.Name": "Sam" },
    { "_id": 0, "Details.$": 1 }
);

{ "Details": [ { "Name": "Sam", "Age": 23 } ] }

Key Points

• Use dot notation ("arrayField.subfield") to query within subdocuments.
• The $ positional operator in projection returns only the first matched array element.
• Set _id: 0 to exclude the document ID from results.

Conclusion

Combine dot notation queries with the $ positional operator in projection to select specific subdocuments from arrays. This approach returns only the matched subdocument rather than the entire array.