MongoDB Validation

Why Validate Documents

MongoDB's flexibility means any document, with any fields, can normally be inserted into a collection. That flexibility is powerful during early development, but as an application matures you usually want guarantees: a user document must have an email, an age field must be a positive number, a status field must be one of a fixed set of values. Schema validation adds those guarantees at the database level, so bad data is rejected before it is ever written.

Validation rules are expressed using $jsonSchema, a MongoDB implementation of the JSON Schema standard. You attach the rules to a collection, either when you create it or by updating an existing collection with collMod.

Creating a Collection with Validation

The validator option on createCollection defines the rules. required lists mandatory fields, properties describes the type and constraints for each field.

db.createCollection("users", {
  validator: {
    $jsonSchema: {
      bsonType: "object",
      required: ["name", "email", "age"],
      properties: {
        name: { bsonType: "string", description: "must be a string" },
        email: { bsonType: "string", pattern: "^.+@.+$" },
        age: { bsonType: "int", minimum: 0, maximum: 120 }
      }
    }
  }
})

Adding Validation to an Existing Collection

If the collection already exists, use collMod to attach or change its validator without recreating it.

db.runCommand({
  collMod: "users",
  validator: {
    $jsonSchema: {
      bsonType: "object",
      required: ["name", "email"],
      properties: {
        status: { enum: ["active", "inactive", "banned"] }
      }
    }
  }
})

Controlling Strictness

Two options control how validation behaves: validationLevel decides which documents get checked (strict checks all inserts and updates, moderate only checks documents that were already valid), and validationAction decides what happens on failure (error rejects the write, warn logs a warning but still allows it).

db.runCommand({
  collMod: "users",
  validationLevel: "moderate",
  validationAction: "warn"
})

• required — array of field names that must be present.
• properties — per-field type and constraint rules (bsonType, minimum, maximum, pattern, enum).
• validationLevel — "strict" (default) or "moderate".
• validationAction — "error" (default, rejects the write) or "warn" (logs only).