MongoDB aggregate to get the count of field values of corresponding duplicate names?

To count field values for corresponding duplicate names in MongoDB, use the aggregation pipeline with $unwind, $group, and $project stages to flatten arrays, group by names, and calculate counts.

Syntax

db.collection.aggregate([
    { $unwind: "$arrayField" },
    { $group: { 
        _id: "$arrayField.nameField", 
        count: { $sum: 1 },
        otherFields: { $push: "$arrayField.otherField" }
    }},
    { $project: { 
        count: 1, 
        otherFieldCount: { $size: "$otherFields" }
    }}
]);

Sample Data

db.demo558.insertOne({
    _id: 100,
    CountryCode: 101,
    details: [
        {
            Name: "Chris",
            Subject: "MySQL"
        },
        {
            Name: "Chris",
            Subject: "MongoDB"
        },
        {
            Name: "Chris",
            Subject: "Java"
        },
        {
            Name: "Bob",
            Subject: "Python"
        },
        {
            Name: "Bob",
            Subject: "Java"
        }
    ]
});

{ "acknowledged": true, "insertedId": 100 }

View Sample Data

db.demo558.find();

{
    "_id": 100,
    "CountryCode": 101,
    "details": [
        { "Name": "Chris", "Subject": "MySQL" },
        { "Name": "Chris", "Subject": "MongoDB" },
        { "Name": "Chris", "Subject": "Java" },
        { "Name": "Bob", "Subject": "Python" },
        { "Name": "Bob", "Subject": "Java" }
    ]
}

Count Duplicate Names and Subjects

db.demo558.aggregate([
    { $unwind: "$details" },
    { $group: { 
        _id: "$details.Name", 
        NameCount: { $sum: 1 }, 
        Subject: { $push: "$details.Subject" }
    }},
    { $project: { 
        NameCount: 1, 
        SubjectCount: { $size: "$Subject" }
    }}
]);

{ "_id": "Bob", "NameCount": 2, "SubjectCount": 2 }
{ "_id": "Chris", "NameCount": 3, "SubjectCount": 3 }

How It Works

• $unwind: Flattens the details array, creating separate documents for each array element
• $group: Groups by Name field, counts occurrences, and collects all subjects into an array
• $project: Displays the name count and calculates subject count using $size

Conclusion

Use aggregation pipeline with $unwind to flatten arrays, $group to count duplicates by name, and $project with $size to count associated field values. This approach efficiently handles nested array data for duplicate analysis.