Schema Validation

MongoDB's flexible schema is a strength, but real applications usually still want some guarantees — a users collection where every document is required to have an email. Schema validation lets you enforce exactly the rules you choose, and no more.

An isometric illustration of a document passing through a checkmark-shaped gate, with a rejected document bouncing off a red X-shaped gate beside it

Creating a Collection with Validation

db.createCollection("users", {
  validator: {
    $jsonSchema: {
      bsonType: "object",
      required: ["name", "email"],
      properties: {
        name: { bsonType: "string" },
        email: { bsonType: "string", pattern: "^.+@.+$" },
        age: { bsonType: "int", minimum: 0 }
      }
    }
  }
})

What Happens on a Violation

Trying to insert a document that breaks a required rule fails with a validation error instead of being saved:

db.users.insertOne({ name: "Test User" })
// Error: Document failed validation — "email" is required

Adding Validation to an Existing Collection

collMod applies a validator to a collection that already exists:

db.runCommand({
  collMod: "users",
  validator: {
    $jsonSchema: {
      required: ["email"]
    }
  }
})

Existing documents are not checked retroactively

By default, validation applies to new inserts and updates but does not retroactively check documents already in the collection — existing data that would fail the new rule stays exactly as it is unless it is updated or you set validationLevel: "strict".