How to retrieve all nested fields from MongoDB collection?

To retrieve all nested fields from a MongoDB collection, use the $unwind aggregation operator to flatten arrays, then $project to extract specific nested fields into the document structure.

Syntax

db.collection.aggregate([
    { $unwind: "$arrayFieldName" },
    { $project: { 
        "field1": "$arrayFieldName.nestedField1",
        "field2": "$arrayFieldName.nestedField2"
    }}
]);

Sample Data

db.demo138.insertMany([
    {
        "Id": 101,
        "PlayerDetails": [
            {"PlayerName": "Chris", "PlayerScore": 400},
            {"PlayerName": "David", "PlayerScore": 1000}
        ]
    },
    {
        "Id": 102,
        "PlayerDetails": [
            {"PlayerName": "Bob", "PlayerScore": 500},
            {"PlayerName": "Carol", "PlayerScore": 600}
        ]
    }
]);

{
    "acknowledged": true,
    "insertedIds": {
        "0": ObjectId("5e31bb9ffdf09dd6d08539a1"),
        "1": ObjectId("5e31bbcefdf09dd6d08539a2")
    }
}

View Sample Data

db.demo138.find();

{
    "_id": ObjectId("5e31bb9ffdf09dd6d08539a1"), "Id": 101, "PlayerDetails": [
        { "PlayerName": "Chris", "PlayerScore": 400 },
        { "PlayerName": "David", "PlayerScore": 1000 }
    ]
}
{
    "_id": ObjectId("5e31bbcefdf09dd6d08539a2"), "Id": 102, "PlayerDetails": [
        { "PlayerName": "Bob", "PlayerScore": 500 },
        { "PlayerName": "Carol", "PlayerScore": 600 }
    ]
}

Retrieve All Nested Fields

db.demo138.aggregate([
    { $unwind: "$PlayerDetails" },
    { $project: { 
        "PlayerName": "$PlayerDetails.PlayerName",
        "PlayerScore": "$PlayerDetails.PlayerScore"
    }}
]);

{ "_id": ObjectId("5e31bb9ffdf09dd6d08539a1"), "PlayerName": "Chris", "PlayerScore": 400 }
{ "_id": ObjectId("5e31bb9ffdf09dd6d08539a1"), "PlayerName": "David", "PlayerScore": 1000 }
{ "_id": ObjectId("5e31bbcefdf09dd6d08539a2"), "PlayerName": "Bob", "PlayerScore": 500 }
{ "_id": ObjectId("5e31bbcefdf09dd6d08539a2"), "PlayerName": "Carol", "PlayerScore": 600 }

How It Works

• $unwind creates separate documents for each array element
• $project extracts nested fields to the top level of each document
• Each nested object becomes an individual document with the specified fields

Conclusion

Use $unwind followed by $project in aggregation pipelines to flatten nested arrays and extract specific fields. This transforms nested data structures into individual documents for easier querying and analysis.