Chapter 107 — MERN Interview: MongoDB & Mongoose Questions

📖 How to Use This Chapter

The MongoDB questions most asked in MERN interviews, focused on how you use it in real projects with Mongoose. For a full MongoDB question bank (70+ questions and 25 query problems), see the MongoDB section.

1. Why MongoDB in a MERN app?

JSON-like documents map directly to JavaScript objects and API responses, flexible schema for evolving features, rich queries and aggregation, and built-in replication and sharding. For heavily relational, transaction-heavy data (accounting), PostgreSQL can be a better fit.

2. How did you design the schema in your project?

Explain one real example with embed vs reference:

"Orders embed their line items — product name, price and quantity at purchase time — because items are always read with the order and must not change if the product price changes later. Orders reference the user by userId, because users are read independently and have many orders. Reviews are a separate collection referencing productId, because a product can have thousands."

3. Embed vs reference — rules?

Embed: small, bounded, read together, owned by the parent (address, line items). Reference: large or unbounded, shared, or accessed independently (users, reviews, logs). Max document size is 16 MB.

4. What is Mongoose and why use it?

An ODM for Node: schemas, validation, defaults, middleware (hooks), populate, model methods. It gives structure to a flexible database.

5. Show a schema with validation, hooks and methods.

const userSchema = new mongoose.Schema({
  name: { type: String, required: true, trim: true },
  email: { type: String, required: true, unique: true, lowercase: true },
  password: { type: String, required: true, minlength: 8, select: false },
  role: { type: String, enum: ["user", "admin"], default: "user" },
}, { timestamps: true });

userSchema.pre("save", async function () {
  if (this.isModified("password")) this.password = await bcrypt.hash(this.password, 12);
});

userSchema.methods.comparePassword = function (plain) {
  return bcrypt.compare(plain, this.password);
};

6. populate vs $lookup?

populate runs an extra query from Mongoose and replaces ids with documents — simple. $lookup is a database-side join in an aggregation — needed when you filter/sort/group on joined fields or want one round trip.

7. How do indexes help? How did you use them?

Indexes avoid scanning the whole collection. Example: the order list filters by userId and status and sorts by createdAt → compound index { userId: 1, status: 1, createdAt: -1 } (Equality, Sort, Range rule). Verify with .explain("executionStats") — look for IXSCAN instead of COLLSCAN.

8. How do you find and fix a slow query?

Enable the profiler / check slow logs or Atlas Performance Advisor → run explain → add or fix the index → reduce fields with projection → use .lean() for read-only data → paginate → re-measure.

9. What does .lean() do?

Returns plain JavaScript objects instead of Mongoose documents — faster and lighter, but no save(), virtuals or methods. Use for read-only API responses.

10. Why don't validators run on findByIdAndUpdate?

Mongoose runs schema validators on save() by default, not on update queries. Pass { runValidators: true, new: true }. Also note pre("save") hooks don't run on updateOne.

11. How do you prevent two users buying the last item?

Atomic conditional update:

const updated = await Product.findOneAndUpdate(
  { _id: productId, stock: { $gte: qty } },
  { $inc: { stock: -qty } },
  { new: true },
);
if (!updated) throw new AppError(409, "Out of stock");

12. Have you used transactions? When?

When several documents must change together — e.g. create an order and decrement stock for several products and record a payment. Requires a replica set (Atlas is one).

await mongoose.connection.transaction(async (session) => {
  await Order.create([{ userId, items, total }], { session });
  for (const i of items) {
    const ok = await Product.updateOne({ _id: i.productId, stock: { $gte: i.qty } }, { $inc: { stock: -i.qty } }, { session });
    if (!ok.modifiedCount) throw new Error("Out of stock");
  }
});

13. Write an aggregation for monthly revenue.

Order.aggregate([
  { $match: { status: "paid", createdAt: { $gte: new Date("2026-01-01") } } },
  { $group: { _id: { $dateToString: { format: "%Y-%m", date: "$createdAt" } }, revenue: { $sum: "$total" }, orders: { $sum: 1 } } },
  { $sort: { _id: 1 } },
]);

14. Top 5 customers by spend with names?

Order.aggregate([
  { $match: { status: "paid" } },
  { $group: { _id: "$userId", spent: { $sum: "$total" } } },
  { $sort: { spent: -1 } },
  { $limit: 5 },
  { $lookup: { from: "users", localField: "_id", foreignField: "_id", as: "user" } },
  { $project: { _id: 0, name: { $first: "$user.name" }, spent: 1 } },
]);

15. How do you implement search?

Simple: regex anchored at the start or a text index ($text). Better: MongoDB Atlas Search for fuzzy, autocomplete and relevance. Always escape user input used in regex.

16. How do you implement soft delete?

Add deletedAt: Date | null; set it instead of deleting; filter deletedAt: null in queries (Mongoose query middleware can add it automatically); a partial index keeps it fast.

17. How do you handle a duplicate key error?

Error code 11000 from a unique index (e.g. email). Catch it in the error handler and return 409 Conflict with a clear message.

18. How do you prevent NoSQL injection?

Validate types (email must be a string — not { "$ne": null }), don't pass req.body directly into filters, use mongoose.set("sanitizeFilter", true), and keep strictQuery on.

19. What is a replica set and why does it matter to you as a developer?

A primary plus secondaries with automatic failover. It's needed for transactions and change streams; use w: "majority" (default) so writes survive failover; reading from secondaries can return slightly old data.

20. How do you connect Mongoose correctly in production?

One connection per process at start-up (not per request), with pool size, timeouts, and retry handling; close it on shutdown. In serverless, cache the connection in a module-level variable.

21. How do you migrate schema changes?

Make the code handle old and new shapes, run a batched migration script (updateMany in chunks), then remove old-code paths. Add a schemaVersion field for big changes.

22. What are TTL indexes used for?

Auto-deleting documents after a time: OTPs, sessions, password reset tokens, temporary logs.

otpSchema.index({ createdAt: 1 }, { expireAfterSeconds: 300 });

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