Find a strict document that contains only a specific field with a fixed length?

To find documents containing only specific fields with a fixed length in MongoDB, use the $exists operator combined with $where to check field count. This ensures the document has exactly the required fields and no additional ones.

Syntax

db.collection.find({
    "field1": { $exists: true },
    "field2": { $exists: true },
    $where: function() { return Object.keys(this).length === expectedCount }
});

Sample Data

db.veryStrictDocumentDemo.insertMany([
    {"StudentFirstName": "John", "StudentLastName": "Doe", "StudentAge": 23},
    {"StudentFirstName": "Larry"},
    {"StudentFirstName": "David", "StudentLastName": "Miller"},
    {"StudentFirstName": "Chris"},
    {"StudentFirstName": "Bob", "StudentLastName": "Brown"}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cda4bcdb50a6c6dd317adb8"),
        ObjectId("5cda4bdbb50a6c6dd317adb9"),
        ObjectId("5cda4becb50a6c6dd317adba"),
        ObjectId("5cda4bfbb50a6c6dd317adbb"),
        ObjectId("5cda4c6db50a6c6dd317adbc")
    ]
}

Display All Documents

db.veryStrictDocumentDemo.find();

{ "_id" : ObjectId("5cda4bcdb50a6c6dd317adb8"), "StudentFirstName" : "John", "StudentLastName" : "Doe", "StudentAge" : 23 }
{ "_id" : ObjectId("5cda4bdbb50a6c6dd317adb9"), "StudentFirstName" : "Larry" }
{ "_id" : ObjectId("5cda4becb50a6c6dd317adba"), "StudentFirstName" : "David", "StudentLastName" : "Miller" }
{ "_id" : ObjectId("5cda4bfbb50a6c6dd317adbb"), "StudentFirstName" : "Chris" }
{ "_id" : ObjectId("5cda4c6db50a6c6dd317adbc"), "StudentFirstName" : "Bob", "StudentLastName" : "Brown" }

Example: Find Documents with Exactly 4 Fields

To find documents that have exactly StudentFirstName, StudentLastName, and one additional field (besides _id) ?

db.veryStrictDocumentDemo.find({
    "StudentFirstName": { $exists: true },
    "StudentLastName": { $exists: true },
    $where: function() { return Object.keys(this).length === 4 }
});

{ "_id" : ObjectId("5cda4bcdb50a6c6dd317adb8"), "StudentFirstName" : "John", "StudentLastName" : "Doe", "StudentAge" : 23 }

Key Points

• $exists: true ensures required fields are present
• $where with Object.keys(this).length validates exact field count
• The count includes the _id field (MongoDB automatically adds it)
• This approach filters documents with extra unwanted fields

Conclusion

Use $exists operators for required fields and $where with Object.keys().length to find documents with an exact field count. This ensures strict document structure validation in MongoDB queries.