How to get items from an object array in MongoDB?

To get items from an object array in MongoDB, use the aggregation pipeline with $unwind to flatten arrays, $match to filter specific documents, and $group to collect the desired items.

Syntax

db.collection.aggregate([
    { $unwind: "$arrayField" },
    { $match: { "arrayField.field": { $in: ["value1", "value2"] } } },
    { $group: { _id: null, result: { $addToSet: "$arrayField.targetField" } } }
]);

Sample Data

db.demo459.insertOne({
    "_id": 1,
    "Information": [
        {
            "Name": "Chris",
            "_id": ObjectId(),
            "details": ["HR"]
        },
        {
            "Name": "David",
            "_id": ObjectId(),
            "details": ["Developer"]
        },
        {
            "Name": "Bob",
            "_id": ObjectId(),
            "details": ["Account"]
        }
    ]
});

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

Verify Sample Data

db.demo459.find();

{
    "_id": 1,
    "Information": [
        { "Name": "Chris", "_id": ObjectId("5e7ef4a7dbcb9adb296c95c9"), "details": ["HR"] },
        { "Name": "David", "_id": ObjectId("5e7ef4a7dbcb9adb296c95ca"), "details": ["Developer"] },
        { "Name": "Bob", "_id": ObjectId("5e7ef4a7dbcb9adb296c95cb"), "details": ["Account"] }
    ]
}

Example: Extract Details for Chris and Bob

db.demo459.aggregate([
    { $unwind: "$Information" },
    { $unwind: "$Information.details" },
    { $match: { "Information.Name": { $in: ["Chris", "Bob"] } } },
    { $group: { _id: null, detailList: { $addToSet: "$Information.details" } } }
]);

{ "_id": null, "detailList": ["Account", "HR"] }

How It Works

• $unwind: "$Information" − Flattens the Information array into separate documents
• $unwind: "$Information.details" − Flattens the nested details array
• $match − Filters documents where Name is either "Chris" or "Bob"
• $group − Collects unique detail values using $addToSet

Conclusion

Use MongoDB's aggregation pipeline with $unwind, $match, and $group to extract specific items from object arrays. This approach efficiently filters and collects data from nested array structures.