Check for existing documents/embedded documents in MongoDB

To check for existing documents or embedded documents in MongoDB, use the $exists operator. This operator matches documents where a specified field exists, regardless of its value.

Syntax

db.collection.find({ "fieldName": { $exists: true } })
db.collection.find({ "embeddedField.subField": { $exists: true } })

Sample Data

Let us create a collection with documents containing embedded arrays ?

db.demo322.insertMany([
    {
        "id": 1001,
        "details": [
            { "Score": 10000, "Name": "Bob" },
            { "Score": 98000, "Name": "Sam" }
        ]
    },
    {
        "id": 10002,
        "details": [
            { "Score": 9000 },
            { "Score": 91000 }
        ]
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e5113e2f8647eb59e56206c"),
        ObjectId("5e5113faf8647eb59e56206d")
    ]
}

Display all documents from the collection ?

db.demo322.find();

{
    "_id": ObjectId("5e5113e2f8647eb59e56206c"),
    "id": 1001,
    "details": [
        { "Score": 10000, "Name": "Bob" },
        { "Score": 98000, "Name": "Sam" }
    ]
}
{
    "_id": ObjectId("5e5113faf8647eb59e56206d"),
    "id": 10002,
    "details": [
        { "Score": 9000 },
        { "Score": 91000 }
    ]
}

Example: Check for Embedded Field Existence

Check if any documents contain the "Name" field within the "details" array ?

db.demo322.find({ "details.Name": { $exists: true } }).count() > 0;

true

This returns true because the first document has "Name" fields in its details array.

Key Points

• $exists: true matches documents where the field exists
• $exists: false matches documents where the field does not exist
• Use dot notation to check embedded fields in arrays or subdocuments
• Combine with .count() to verify existence of matching documents

Conclusion

The $exists operator is essential for checking field presence in MongoDB documents. Use dot notation to target embedded fields within arrays or subdocuments for precise field existence validation.