Extract subarray value in MongoDB?

To extract a subarray value in MongoDB, you can use the $elemMatch projection operator to return only the first array element that matches the specified condition.

Syntax

db.collection.find(
    { query },
    { arrayField: { $elemMatch: { field: "value" } } }
);

Sample Data

Let us first create a collection with documents ?

db.extractSubArrayDemo.insertOne({
    _id: 101,
    "clientName": "Larry",
    "ClientDetails": [
        {
            "ClientProjectName": "Online Game",
            "DeveloperTeamSize": 10
        },
        {
            "ClientProjectName": "Pig Dice Game",
            "DeveloperTeamSize": 12
        },
        {
            "ClientProjectName": "Web Student Tracker",
            "DeveloperTeamSize": 11
        }
    ]
});

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

Display all documents from the collection ?

db.extractSubArrayDemo.find().pretty();

{
    "_id": 101,
    "clientName": "Larry",
    "ClientDetails": [
        {
            "ClientProjectName": "Online Game",
            "DeveloperTeamSize": 10
        },
        {
            "ClientProjectName": "Pig Dice Game",
            "DeveloperTeamSize": 12
        },
        {
            "ClientProjectName": "Web Student Tracker",
            "DeveloperTeamSize": 11
        }
    ]
}

Example: Extract Specific Array Element

Extract only the "Pig Dice Game" project details from the ClientDetails array ?

db.extractSubArrayDemo.find(
    { '_id': 101 },
    { 
        _id: 0, 
        ClientDetails: { $elemMatch: { ClientProjectName: 'Pig Dice Game' } }
    }
).pretty();

{
    "ClientDetails": [
        {
            "ClientProjectName": "Pig Dice Game",
            "DeveloperTeamSize": 12
        }
    ]
}

Key Points

• $elemMatch returns only the first matching array element.
• The result still wraps the matched element in an array format.
• Use _id: 0 to exclude the document ID from results.

Conclusion

The $elemMatch projection operator effectively extracts specific subarray elements based on field conditions. It's particularly useful when you need only one matching element from large arrays instead of the entire array.