MongoDB query to remove empty objects in an object-array?

To remove empty objects from an object-array in MongoDB, use the $pull operator combined with $exists: false to match objects that lack any fields. This effectively removes objects with no key-value pairs.

Syntax

db.collection.update(
    {},
    { "$pull": { "arrayField": { "fieldName": { "$exists": false } } } },
    { "multi": true }
);

Sample Data

Let us create a collection with documents containing empty objects in the array ?

db.removeEmptyObjectsDemo.insertOne({
    "_id": 101,
    "LoginDate": new ISODate(),
    "UserDetails": [
        {
            "UserName": "John"
        },
        {
        },
        {
            "UserName": "Sam"
        }
    ]
});

{ "acknowledged": true, "insertedId": 101 }

Display the document to verify the data structure ?

db.removeEmptyObjectsDemo.find().pretty();

{
    "_id": 101,
    "LoginDate": ISODate("2019-05-25T04:46:29.505Z"),
    "UserDetails": [
        {
            "UserName": "John"
        },
        {
        },
        {
            "UserName": "Sam"
        }
    ]
}

Remove Empty Objects

Use $pull with $exists: false to remove objects that don't contain the specified field ?

db.removeEmptyObjectsDemo.update(
    {},
    { "$pull": { "UserDetails": { "UserName": { "$exists": false } } } },
    { "multi": true }
);

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1 })

Verify Result

Check the updated document to confirm empty objects are removed ?

db.removeEmptyObjectsDemo.find().pretty();

{
    "_id": 101,
    "LoginDate": ISODate("2019-05-25T04:46:29.505Z"),
    "UserDetails": [
        {
            "UserName": "John"
        },
        {
            "UserName": "Sam"
        }
    ]
}

Conclusion

The $pull operator with $exists: false effectively removes empty objects from arrays by targeting objects that lack specific fields. Use multi: true to update all matching documents in the collection.