Implement match and project in MongoDB aggregate

The $match operator filters documents to pass only those that match specified conditions to the next pipeline stage. The $project operator reshapes documents by including, excluding, or adding new fields before passing them to the next stage.

Syntax

db.collection.aggregate([
    { $match: { field: value } },
    { $project: { field1: 1, field2: 0, newField: "$existingField" } }
]);

Sample Data

Let us create a collection with student documents ?

db.demo545.insertMany([
    {
        "Name": "Chris",
        "details": {
            "SubjectScore1": 56,
            "SubjectScore2": 56
        }
    },
    {
        "Name": "David", 
        "details": {
            "SubjectScore1": 78,
            "SubjectScore2": 78
        }
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e8e246e9e5f92834d7f05d5"),
        ObjectId("5e8e24709e5f92834d7f05d6")
    ]
}

Display all documents from the collection ?

db.demo545.find();

{
    "_id": ObjectId("5e8e246e9e5f92834d7f05d5"),
    "Name": "Chris",
    "details": {
        "SubjectScore1": 56,
        "SubjectScore2": 56
    }
}
{
    "_id": ObjectId("5e8e24709e5f92834d7f05d6"),
    "Name": "David",
    "details": {
        "SubjectScore1": 78,
        "SubjectScore2": 78
    }
}

Example: Combining $match, $project, and $group

Use aggregation to filter documents, project specific fields, and group by SubjectScore2 values ?

db.demo545.aggregate([
    { $match: {} },
    { 
        $project: {
            "_id": 0,
            "details.SubjectScore1": 1,
            "out": "$details.SubjectScore2"
        }
    },
    { 
        $group: {
            "_id": "$out"
        }
    }
]);

{ "_id": 78 }
{ "_id": 56 }

How It Works

• $match: {} − Passes all documents (empty filter matches everything)
• $project − Excludes _id, includes SubjectScore1, creates new field out with SubjectScore2 values
• $group − Groups documents by the out field values, creating unique score groups

Conclusion

Combine $match and $project in aggregation pipelines to filter and reshape data efficiently. The $match stage should typically come first to reduce the dataset size early in the pipeline.