MongoDB Aggregate JSON array field for the matching field of other collection?

To aggregate JSON array field for matching fields from another collection in MongoDB, use $lookup combined with $unwind to join collections based on array elements, then reconstruct the result using $group.

Syntax

db.collection1.aggregate([
    { $unwind: "$arrayField" },
    { $lookup: {
        from: "collection2",
        localField: "arrayField.joinField",
        foreignField: "_id",
        as: "matchedData"
    }},
    { $project: { 
        "_id": 1,
        "arrayField.newField": { $arrayElemAt: ["$matchedData.targetField", 0] }
    }},
    { $group: { 
        _id: "$_id",
        arrayField: { $push: "$arrayField" }
    }}
]);

Sample Data

Create the first collection with an array field ?

db.demo101.insertOne({
    "_id": "1",
    "Details": [
        { "PId": "200" },
        { "PId": "201" },
        { "PId": "201" }
    ]
});

{ "acknowledged": true, "insertedId": "1" }

Create the second collection with matching reference data ?

db.demo102.insertMany([
    { "_id": "200", "CustEmailId": "John@gmail.com" },
    { "_id": "201", "CustEmailId": "Carol@gmail.com" }
]);

{ "acknowledged": true, "insertedIds": ["200", "201"] }

Example

Aggregate to replace PId values with corresponding email addresses ?

db.demo101.aggregate([
    { $unwind: "$Details" },
    { $lookup: {
        from: "demo102",
        localField: "Details.PId",
        foreignField: "_id",
        as: "out"
    }},
    { $project: {
        "_id": 1,
        "Details.PId": { $arrayElemAt: ["$out.CustEmailId", 0] }
    }},
    { $group: {
        _id: "$_id",
        Details: { $push: "$Details" }
    }}
]);

{
    "_id": "1",
    "Details": [
        { "PId": "John@gmail.com" },
        { "PId": "Carol@gmail.com" },
        { "PId": "Carol@gmail.com" }
    ]
}

How It Works

• $unwind deconstructs the Details array into separate documents
• $lookup joins with demo102 collection based on PId matching _id
• $project replaces PId with the corresponding email using $arrayElemAt
• $group reconstructs the array structure by grouping back by _id

Conclusion

Use $unwind to flatten arrays, $lookup to join collections, and $group to rebuild the structure. This pipeline effectively replaces array field values with corresponding data from another collection.