MongoDB - How to check for equality in collection and in embedded document?

To check for equality between a field in the main collection and a field in an embedded document, use the $where operator with JavaScript expressions. This allows you to compare values across different nesting levels within the same document.

Syntax

db.collection.find({
    $where: 'this.fieldName === this.embeddedDoc.fieldName'
});

Sample Data

db.demo292.insertMany([
    {
        "FirstName": "Chris",
        "LastName": "Brown",
        "Friend": {
            "FirstName": "David",
            "LastName": "Miller"
        }
    },
    {
        "FirstName": "John",
        "LastName": "Doe",
        "Friend": {
            "FirstName": "Mike",
            "LastName": "Doe"
        }
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e4c10aa5d93261e4bc9ea30"),
        ObjectId("5e4c10dc5d93261e4bc9ea31")
    ]
}

View Sample Data

db.demo292.find();

{
    "_id": ObjectId("5e4c10aa5d93261e4bc9ea30"),
    "FirstName": "Chris",
    "LastName": "Brown",
    "Friend": {
        "FirstName": "David",
        "LastName": "Miller"
    }
}
{
    "_id": ObjectId("5e4c10dc5d93261e4bc9ea31"),
    "FirstName": "John",
    "LastName": "Doe",
    "Friend": {
        "FirstName": "Mike",
        "LastName": "Doe"
    }
}

Example: Find Documents Where LastName Equals Friend's LastName

db.demo292.find({
    $where: 'this.Friend.LastName === this.LastName'
});

{
    "_id": ObjectId("5e4c10dc5d93261e4bc9ea31"),
    "FirstName": "John",
    "LastName": "Doe",
    "Friend": {
        "FirstName": "Mike",
        "LastName": "Doe"
    }
}

Key Points

• Use $where with JavaScript expressions to compare fields across different document levels.
• Access embedded document fields using dot notation: this.embeddedDoc.field
• $where evaluates JavaScript expressions for each document, making it slower than other operators.

Conclusion

The $where operator enables equality checks between collection fields and embedded document fields using JavaScript expressions. While powerful for complex comparisons, use it judiciously as it can impact query performance.