Validations & Transactions

Modern applications require data integrity. MongoDB provides Schema Validation to enforce document structure and Transactions for ACID compliance.

1. Schema Validation

MongoDB allows you to define JSON Schema validation rules for your collections. Documents that don't match the rules will be rejected.

db.createCollection("students", {
  validator: {
    $jsonSchema: {
      bsonType: "object",
      required: ["name", "year", "major"],
      properties: {
        name: { bsonType: "string", description: "must be a string and is required" },
        year: { bsonType: "int", minimum: 2017, maximum: 3017, description: "must be an integer in [ 2017, 3017 ] and is required" },
        major: { enum: ["Math", "English", "Computer Science"], description: "can only be one of the enum values and is required" }
      }
    }
  }
})

2. Multi-Document Transactions

Transactions allow you to perform multiple operations (on one or more collections) as a single atomic unit. If any part fails, the entire transaction is rolled back.

// Example Transaction using Node.js Driver
const session = client.startSession();

try {
  await session.withTransaction(async () => {
    // Operation 1: Deduct balance
    await db.collection("accounts").updateOne(
      { name: "A" }, { $inc: { balance: -100 } }, { session }
    );
    
    // Operation 2: Add balance
    await db.collection("accounts").updateOne(
      { name: "B" }, { $inc: { balance: 100 } }, { session }
    );
  });
} catch (error) {
  console.log("Transaction aborted due to error:", error);
} finally {
  await session.endSession();
}

3. Validation Levels & Actions

• validationLevel: Defines how strictly MongoDB applies validation rules to existing documents (off, strict, moderate).
• validationAction: Determines whether MongoDB should error (reject) or warn but allow the invalid document.