Adding a Validator to a Collection
Learners will attach a JSON Schema validator using createCollection and collMod to enforce required fields and data types.
Adding a Validator to a Collection is a free MongoDB Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the MongoDB Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Schema Validation Matters
MongoDB is schema-flexible by default—any document can be inserted regardless of its shape. While this is useful in development, production databases need guardrails to prevent malformed data. MongoDB's schema validation feature lets you attach a JSON Schema rule set to a collection so that only well-formed documents can be inserted or updated, catching data quality problems at the database layer before they propagate.
JSON Schema as the Validation Language
MongoDB uses the industry-standard JSON Schema specification (draft 4) to express validation rules. You define a $jsonSchema object that declares which fields are required, what types they must be, and any additional constraints. The same format is used everywhere JSON Schema appears—in OpenAPI specs, form libraries, and now MongoDB validators.
Adding a Validator at Collection Creation
Pass a validator option when calling db.createCollection(). The validator contains a $jsonSchema document. The example below requires every users document to have a name (string) and an email (string), and optionally an age (integer).
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', description: 'must be a string and is required' },
age: { bsonType: 'int', minimum: 0, description: 'optional, must be a non-negative int' }
}
}
}
});Adding a Validator to an Existing Collection
Use the collMod (collection modification) command to attach or update a validator on a collection that already exists and may already contain data. This does not validate existing documents by default—it only applies to future writes unless you also change validationLevel.
db.runCommand({
collMod: 'users',
validator: {
$jsonSchema: {
bsonType: 'object',
required: ['name', 'email'],
properties: {
name: { bsonType: 'string' },
email: { bsonType: 'string' }
}
}
}
});What Happens When Validation Fails
By default, if a document violates the validator, MongoDB rejects the write and throws an error: Document failed validation. The error includes a details field explaining exactly which rule was broken, making it easy to diagnose and fix the offending document. The insert or update is rolled back completely—no partial writes occur.
// This insert violates the validator — email is missing
try {
db.users.insertOne({ name: 'Bob' });
} catch (err) {
console.error(err.errInfo.details);
// Output: required field 'email' is missing
}Viewing the Current Validator
To inspect the validator attached to a collection, use db.getCollectionInfos() or query the system.js namespace. The returned document includes the full validator specification under options.validator, allowing you to review, copy, or compare validators across environments.
// List all collections and their options, including validators
const info = db.getCollectionInfos({ name: 'users' });
console.log(JSON.stringify(info[0].options.validator, null, 2));Removing a Validator
To remove all validation from a collection, run collMod with an empty validator object. This returns the collection to its default schema-free state. You might do this temporarily during a bulk data migration or permanently when retiring validation in favour of application-layer checks.
// Remove the validator entirely
db.runCommand({
collMod: 'users',
validator: {}
});Validation in Mongoose vs Native MongoDB
Mongoose has its own schema validation at the ODM layer that runs in JavaScript before sending data to MongoDB. However, Mongoose validation can be bypassed with insertMany or direct driver calls. Adding a JSON Schema validator at the database level creates an additional safety net that no client can bypass, regardless of the driver or language used.
Nested Object Validation
JSON Schema validators can reach into embedded sub-documents. Use the properties key to define rules for nested fields, and mark the nested object itself with bsonType: 'object'. This allows you to validate every level of a hierarchical document.
db.createCollection('orders', {
validator: {
$jsonSchema: {
bsonType: 'object',
required: ['customerId', 'shippingAddress'],
properties: {
customerId: { bsonType: 'objectId' },
shippingAddress: {
bsonType: 'object',
required: ['street', 'city'],
properties: {
street: { bsonType: 'string' },
city: { bsonType: 'string' }
}
}
}
}
}
});Array Item Validation
To validate that an array field contains only documents of a specific shape, use items inside the property definition. Each element of the array will be validated against the items schema. This is useful for enforcing the shape of embedded line items, tags, or address arrays.
db.createCollection('carts', {
validator: {
$jsonSchema: {
bsonType: 'object',
properties: {
items: {
bsonType: 'array',
items: {
bsonType: 'object',
required: ['productId', 'qty'],
properties: {
productId: { bsonType: 'objectId' },
qty: { bsonType: 'int', minimum: 1 }
}
}
}
}
}
}
});Schema Validation in Atlas
MongoDB Atlas provides a graphical interface for building and editing collection validators without writing raw JSON. Under the collection's Schema tab you can add properties, set types, and mark required fields through a form. Atlas also shows a validation score—the percentage of existing documents that pass the current schema—helping you measure data quality before enforcing strict validation.
Quick Check
Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.
Lesson Recap
In this lesson you learned: JSON Schema validators are attached via createCollection or collMod, validation failures reject the write and return a detailed error, and nested objects and arrays can also be validated within the same schema document. Next up we explore type, required, and enum constraints in depth.
Frequently asked questions
Is the “Adding a Validator to a Collection” lesson free?
Yes — the full text of “Adding a Validator to a Collection” is free to read here on the web, and the MongoDB Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the MongoDB Academy course, upgrade to CoddyKit PRO.
What will I learn in “Adding a Validator to a Collection”?
Learners will attach a JSON Schema validator using createCollection and collMod to enforce required fields and data types. You practise MongoDB Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start MongoDB Academy?
No prior experience is required. MongoDB Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Adding a Validator to a Collection” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this MongoDB Academy lesson?
Yes. Every MongoDB Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Adding a Validator to a Collection
- Type, Required, and Enum Constraints
- Validation Levels and Actions
- Evolving Schemas Without Downtime