mongodb-coding

MongoDB Coding Standards

Collection Naming

• camelCase, descriptive, plural where appropriate
• Examples: orders, orderItems, userSessions, auditLogs

Document Design

Embed vs Reference Decision

Pattern	Use When	Example
Embed (subdocument)	1:1 or 1:few, data always read together	Order → items array
Reference (ObjectId)	1:many, many:many, data read independently	Order → customerId ref
Bucket	Time-series, high-volume events	Group events by hour/day
Outlier	Mostly small arrays, rare large ones	Most users have <10 addresses

Type Discriminator Pattern

// Single collection for polymorphic types
{ type: "creditCard", cardNumber: "****1234", expiry: "12/26" }
{ type: "bankTransfer", accountNumber: "****5678", routingNumber: "****9012" }

Schema Validation

• Always define validators in production
• Enforce required fields, types, and enums
• Use additionalProperties: false cautiously (breaks additive migrations)

Aggregation Pipelines

db.orders.aggregate([
  { $match: { status: { $in: ["confirmed", "shipped"] } } },
  { $unwind: "$items" },
  { $group: {
      _id: "$items.productId",
      totalQuantity: { $sum: "$items.quantity" },
      totalRevenue: { $sum: { $multiply: ["$items.price", "$items.quantity"] } }
  }},
  { $sort: { totalRevenue: -1 } },
  { $limit: 10 }
]);

Pipeline Best Practices

• $match as early as possible to reduce documents in pipeline
• $project after $match to reduce document size
• Use $lookup with let and pipeline for complex joins
• Avoid $group on unindexed fields

Indexing

Compound Index (ESR Rule)

// Equality, Sort, Range — in that order
db.orders.createIndex({ status: 1, createdAt: -1, total: 1 });

Text Index

db.products.createIndex({ name: "text", description: "text" });

TTL Index

db.sessions.createIndex({ expiresAt: 1 }, { expireAfterSeconds: 0 });

Transactions

const session = client.startSession();
try {
  session.startTransaction();
  await orders.insertOne(order, { session });
  await inventory.updateOne(
    { productId: order.items[0].productId },
    { $inc: { quantity: -order.items[0].quantity } },
    { session }
  );
  await session.commitTransaction();
} catch (error) {
  await session.abortTransaction();
  throw error;
} finally {
  session.endSession();
}

Change Streams

const changeStream = orders.watch([
  { $match: { operationType: { $in: ["insert", "update"] } } }
]);
changeStream.on("change", (change) => {
  console.log("Order changed:", change.documentKey._id);
});

Bulk Operations

const ops = items.map(item => ({
  updateOne: {
    filter: { _id: item._id },
    update: { $set: { price: item.newPrice } },
    upsert: false
  }
}));
await collection.bulkWrite(ops, { ordered: false });