MongoDB Academy · Lesson

Validation Levels and Actions

Learners will configure validationLevel (strict vs moderate) and validationAction (error vs warn) to control how violations are handled.

Validation Levels and Actions is a free MongoDB Academy lesson on CoddyKit — lesson 3 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.

Controlling How Strict Validation Is

MongoDB gives you two independent knobs to tune schema validation behaviour: validationLevel controls which documents are subject to validation, while validationAction controls what happens when a document fails validation. Together they let you introduce validation gradually into existing collections without breaking current data or applications.

validationLevel: strict

strict is the default validation level. In strict mode, every insert and every update must pass the validator—no exceptions. If an existing document in the collection already violates the schema, attempting to update it will still be checked against the validator. Strict mode gives you the strongest data quality guarantee but can be disruptive when applied to a collection with legacy non-conforming documents.

db.runCommand({
  collMod: 'users',
  validator: { $jsonSchema: { /* ... */ } },
  validationLevel: 'strict'   // default — all inserts and updates must pass
});

validationLevel: moderate

moderate level applies the validator only to new inserts and to updates of documents that already pass the validator. Existing non-conforming documents can still be updated without being forced to comply. This is a safe migration path: you enable the validator on a live collection without breaking updates to old documents that don't yet match the new schema.

db.runCommand({
  collMod: 'users',
  validator: { $jsonSchema: { /* ... */ } },
  validationLevel: 'moderate'  // existing non-conforming docs can still be updated
});

validationLevel: off

Setting validationLevel to off disables validation entirely, even if a validator is attached to the collection. This is useful during bulk data migrations or emergency hotfixes where you need to write non-conforming data temporarily. Always re-enable validation after the migration window closes.

// Temporarily disable validation for a migration window
db.runCommand({
  collMod: 'users',
  validationLevel: 'off'
});

// ... run migration ...

// Re-enable strict validation
db.runCommand({
  collMod: 'users',
  validationLevel: 'strict'
});

validationAction: error

error is the default validation action. When a document fails validation, MongoDB rejects the write entirely and returns an error to the client. The document is not written. This is the safest setting for production because it prevents malformed data from ever entering the collection.

db.runCommand({
  collMod: 'orders',
  validator: { $jsonSchema: { /* ... */ } },
  validationAction: 'error'   // default: reject the write, return an error
});

validationAction: warn

warn action allows the document to be written even if it fails validation, but logs a warning message to the MongoDB server log. This is useful during a transition period when you want to observe how many violations occur without blocking existing application traffic. After examining the logs and fixing the offenders, you can switch to error action.

db.runCommand({
  collMod: 'legacy_collection',
  validator: { $jsonSchema: { /* ... */ } },
  validationAction: 'warn'   // write succeeds, violation logged to server log
});

// The server log will show:
// [conn1] Document failed validation: { ... } with schema: { ... }

Combining Level and Action

The two settings compose independently. A common migration strategy is to start with validationLevel: 'moderate' and validationAction: 'warn'—new documents must comply but get logged on failure, while old documents are untouched. After observing the warnings and backfilling old data, switch to strict + error for full enforcement.

// Phase 1: observe without blocking
db.runCommand({
  collMod: 'users',
  validator: { $jsonSchema: { bsonType: 'object', required: ['email'] } },
  validationLevel: 'moderate',
  validationAction: 'warn'
});

// Phase 2 (after backfill): full enforcement
db.runCommand({
  collMod: 'users',
  validationLevel: 'strict',
  validationAction: 'error'
});

Reading the Validation Configuration

Check the current validation settings on a collection using db.getCollectionInfos(). The response includes options.validationLevel and options.validationAction alongside the full validator document. This is useful to audit all collections in a database and verify that production collections have strict enforcement enabled.

const info = db.getCollectionInfos({ name: 'users' });
const opts = info[0].options;
console.log('level:', opts.validationLevel);
console.log('action:', opts.validationAction);
console.log('validator:', JSON.stringify(opts.validator, null, 2));

Bypassing Validation With bypassDocumentValidation

Certain database operations support a bypassDocumentValidation: true option that skips the validator for that specific write. Only users with the bypassDocumentValidation privilege can use this option. It is intended for trusted admin scripts and data migrations only—never use it in application code as it defeats the purpose of having a validator.

// Insert a document bypassing validation — admin/migration use only
db.users.insertOne(
  { name: 'LegacyUser' },  // missing required email
  { bypassDocumentValidation: true }
);

Validation Warnings in the Server Log

When validationAction is warn, each failed validation is written to the MongoDB server log with the WRITE component and WARNING severity. The log line includes the database, collection, the violating document's _id, and the specific schema rule that was broken. You can monitor these logs with Atlas's log viewer or by tailing mongod.log on self-hosted deployments.

Practical Migration Playbook

Here is the recommended four-phase playbook for adding validation to an existing production collection:

Phase 1: Set moderate + warn — no impact, observe violations
Phase 2: Fix application code to send compliant documents
Phase 3: Backfill old non-conforming documents with a migration script
Phase 4: Switch to strict + error — full enforcement

Quick Check

Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.

Lesson Recap

In this lesson you learned: validationLevel (strict/moderate/off) controls which documents are validated, validationAction (error/warn) controls whether failures block or just log, and combining moderate + warn is the safest way to introduce validation on a live collection. Next up we learn how to evolve schemas without downtime on a running MongoDB cluster.

Frequently asked questions

Is the “Validation Levels and Actions” lesson free?

Yes — the full text of “Validation Levels and Actions” 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 “Validation Levels and Actions”?

Learners will configure validationLevel (strict vs moderate) and validationAction (error vs warn) to control how violations are handled. 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 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Validation Levels and Actions” 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.

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