Which characters are NOT allowed in MongoDB field names?

MongoDB field names have specific restrictions. You cannot use the $ symbol at the beginning of field names or the period (.) character anywhere in field names, as these are reserved for MongoDB's internal operations and dot notation.

Syntax

// Invalid field names
{ "$fieldName": "value" }     // Cannot start with $
{ "field.name": "value" }     // Cannot contain dots
{ "field$name": "value" }     // $ allowed in middle/end

// Valid field names  
{ "fieldName": "value" }
{ "field_name": "value" }
{ "Employee Name": "value" }   // Spaces allowed

Prohibited Characters

Example: Valid Field Names

db.charactersAllowedDemo.insertOne({
    "Employee Name": "John",
    "employee_id": 123,
    "salary$amount": 50000,
    "field123": "value"
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5c7fefbc8d10a061296a3c6d")
}

Verify Result

db.charactersAllowedDemo.find().pretty();

{
    "_id": ObjectId("5c7fefbc8d10a061296a3c6d"),
    "Employee Name": "John",
    "employee_id": 123,
    "salary$amount": 50000,
    "field123": "value"
}

Key Points

• $ is allowed in the middle or end of field names, just not at the beginning.
• Spaces and underscores are perfectly valid in field names.
• Numbers and letters are always allowed anywhere in field names.

Conclusion

Avoid starting field names with $ and never use dots anywhere in field names. These restrictions ensure compatibility with MongoDB's query syntax and internal operations.