How to project specific fields from a document inside an array in Mongodb?

To project specific fields from a document inside an array in MongoDB, use the positional operator ($) in the projection parameter. This returns only the first matching array element that satisfies the query condition.

Syntax

db.collection.find(
    { "arrayField.subField": "matchValue" },
    { "arrayField.$": 1 }
);

Sample Data

Let us create a collection with sample documents ?

db.projectSpecificFieldDemo.insertMany([
    {
        "UniqueId": 101,
        "StudentDetails": [
            {"StudentName": "Chris", "StudentCountryName": "US"},
            {"StudentName": "Robert", "StudentCountryName": "UK"}
        ]
    },
    {
        "UniqueId": 102,
        "StudentDetails": [
            {"StudentName": "Robert", "StudentCountryName": "UK"},
            {"StudentName": "David", "StudentCountryName": "AUS"}
        ]
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5ca27aeb6304881c5ce84ba2"),
        ObjectId("5ca27b106304881c5ce84ba3")
    ]
}

View All Documents

db.projectSpecificFieldDemo.find().pretty();

{
    "_id": ObjectId("5ca27aeb6304881c5ce84ba2"),
    "UniqueId": 101,
    "StudentDetails": [
        {
            "StudentName": "Chris",
            "StudentCountryName": "US"
        },
        {
            "StudentName": "Robert",
            "StudentCountryName": "UK"
        }
    ]
}
{
    "_id": ObjectId("5ca27b106304881c5ce84ba3"),
    "UniqueId": 102,
    "StudentDetails": [
        {
            "StudentName": "Robert",
            "StudentCountryName": "UK"
        },
        {
            "StudentName": "David",
            "StudentCountryName": "AUS"
        }
    ]
}

Example: Project Specific Array Element

Find documents where UniqueId is 101 and StudentName is "Chris", then project only the matching array element ?

db.projectSpecificFieldDemo.find(
    { 
        "UniqueId": 101, 
        "StudentDetails.StudentName": "Chris" 
    },
    { 
        "StudentDetails.$": 1 
    }
).pretty();

{
    "_id": ObjectId("5ca27aeb6304881c5ce84ba2"),
    "StudentDetails": [
        {
            "StudentName": "Chris",
            "StudentCountryName": "US"
        }
    ]
}

Key Points

• The $ positional operator returns only the first matching array element.
• The query condition must include a field from the array to match against.
• Use "arrayField.$": 1 in the projection to limit the array to matched elements only.

Conclusion

The positional operator $ effectively projects specific array elements based on query conditions. It returns only the first matching element, making it useful for filtering large arrays to relevant data.