How to validate documents before insert or update in MongoDB?

MongoDB provides built-in document validation to enforce data quality rules before inserting or updating documents. Use the validator option during collection creation to define validation criteria using MongoDB query operators.

Syntax

db.createCollection("collectionName", {
    validator: {
        $jsonSchema: { /* validation rules */ }
        // OR
        /* MongoDB query expression */
    },
    validationLevel: "strict", // or "moderate"
    validationAction: "error"   // or "warn"
});

Example 1: String Type Validation

Create a collection that only accepts documents where FirstName exists and is a string ?

db.createCollection("demo356", {
    validator: {
        $and: [
            {"FirstName": {$type: "string", $exists: true}}
        ]
    }
});

{ "ok" : 1 }

Valid Insert

db.demo356.insertOne({"FirstName": "Chris"});

{
    "acknowledged" : true,
    "insertedId" : ObjectId("5e568d49f8647eb59e5620c7")
}

Invalid Insert (Wrong Type)

db.demo356.insertOne({"FirstName": 909});

WriteError: Document failed validation
{
    "index" : 0,
    "code" : 121,
    "errmsg" : "Document failed validation",
    "op" : {
        "_id" : ObjectId("5e568d52f8647eb59e5620c8"),
        "FirstName" : 909
    }
}

Example 2: JSON Schema Validation

db.createCollection("users", {
    validator: {
        $jsonSchema: {
            bsonType: "object",
            required: ["name", "email"],
            properties: {
                name: {
                    bsonType: "string",
                    description: "must be a string and is required"
                },
                email: {
                    bsonType: "string",
                    pattern: "^.+@.+$",
                    description: "must be a valid email format"
                }
            }
        }
    }
});

Verify Results

db.demo356.find();

{ "_id" : ObjectId("5e568d49f8647eb59e5620c7"), "FirstName" : "Chris" }

Key Points

• Validation occurs during insert and update operations
• Use validationLevel: "moderate" to apply validation only to new/updated documents
• Set validationAction: "warn" to log violations instead of rejecting documents

Conclusion

MongoDB validation ensures data integrity by rejecting documents that don't meet defined criteria. Use $jsonSchema for complex validation or MongoDB query operators for simple type and existence checks.