Mongoose ODM setup in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you connect to MongoDB with Mongoose using this code?

Consider this Express app snippet using Mongoose to connect to MongoDB:

import express from 'express';
import mongoose from 'mongoose';

const app = express();

mongoose.connect('mongodb://localhost:27017/mydb')
  .then(() => console.log('Connected to MongoDB'))
  .catch(err => console.error('Connection error:', err));

app.listen(3000, () => console.log('Server running'));

What will be printed if the MongoDB server is running and accessible?

Express

import express from 'express';
import mongoose from 'mongoose';

const app = express();

mongoose.connect('mongodb://localhost:27017/mydb')
  .then(() => console.log('Connected to MongoDB'))
  .catch(err => console.error('Connection error:', err));

app.listen(3000, () => console.log('Server running'));

A Server running only, no connection messages

B Connected to MongoDB\nServer running

C Connection error: Error: failed to connect\nServer running

D Server running\nConnected to MongoDB

💡 Hint

Promises run asynchronously. The server starts listening immediately, but connection success logs after connection is established.

The app.listen callback runs immediately after starting the server, printing 'Server running'. The mongoose.connect promise resolves after connecting, printing 'Connected to MongoDB'. So the output order is 'Server running' then 'Connected to MongoDB'.

📝 Syntax

intermediate

Which Mongoose schema definition is valid for a User model with required name and optional age?

Choose the correct Mongoose schema code snippet that defines a User model with a required name (string) and an optional age (number).

A const userSchema = new mongoose.Schema({ name: { type: String, required: true }, age: Number });

B const userSchema = new mongoose.Schema({ name: String, required: true, age: Number });

C const userSchema = new mongoose.Schema({ name: String, age: { type: Number, required: false } });

D const userSchema = new mongoose.Schema({ name: { type: String }, age: { type: Number, required: true } });

💡 Hint

Remember that required is a property inside the field's object, not outside.

Option A correctly sets name as an object with type and required. Option A incorrectly places required outside the field definition. Option A makes age required false explicitly but misses name required. Option A makes age required true, which is wrong.

🔧 Debug

advanced

Why does this Mongoose connection code cause an unhandled promise rejection?

Look at this code snippet:

mongoose.connect('mongodb://localhost:27017/mydb');

app.listen(3000, () => console.log('Server running'));

Why might this cause an unhandled promise rejection if the database is down?

Express

mongoose.connect('mongodb://localhost:27017/mydb');

app.listen(3000, () => console.log('Server running'));

A Because app.listen must be called inside the .then() callback of mongoose.connect.

B Because mongoose.connect requires a callback function as second argument to handle errors.

C Because the promise returned by mongoose.connect is not handled with .then() or .catch(), so errors are unhandled.

D Because the connection string is missing the protocol 'mongodb+srv://'.

💡 Hint

Promises must be handled to catch errors.

Calling mongoose.connect returns a promise. If the promise rejects (e.g., DB down) and no .catch() or try/catch handles it, Node.js raises an unhandled promise rejection error. Option C is not required; app.listen can run independently. Option C is outdated; mongoose.connect returns a promise now. Option C is incorrect because 'mongodb://' is valid for local connections.

❓ state_output

advanced

What is the value of user.isNew after saving a new Mongoose document?

Given this code:

const userSchema = new mongoose.Schema({ name: String });
const User = mongoose.model('User', userSchema);

const user = new User({ name: 'Alice' });
console.log(user.isNew);
await user.save();
console.log(user.isNew);

What will be printed by the two console.log statements?

Express

const userSchema = new mongoose.Schema({ name: String });
const User = mongoose.model('User', userSchema);

const user = new User({ name: 'Alice' });
console.log(user.isNew);
await user.save();
console.log(user.isNew);

A false\ntrue

B true\nfalse

C true\ntrue

D false\nfalse

💡 Hint

isNew is true before saving and false after saving.

When a Mongoose document is created but not saved, isNew is true. After calling save(), the document is stored in the database and isNew becomes false.

🧠 Conceptual

expert

Which option best describes Mongoose middleware (hooks) behavior?

Mongoose supports middleware functions called hooks that run before or after certain operations.

Which statement correctly describes how pre and post hooks work in Mongoose?

A Pre hooks run before the operation and can modify the document; post hooks run after and receive the result or error.

B Pre hooks run after the operation completes; post hooks run before the operation starts.

C Pre hooks only run on queries; post hooks only run on document saves.

D Middleware hooks in Mongoose are synchronous only and cannot handle async operations.

💡 Hint

Think about when you want to change data or react to results.

Pre hooks run before the operation and can modify data or cancel the operation. Post hooks run after the operation and receive the result or error. Hooks can be async. Option A reverses the order. Option A is incorrect because hooks can run on queries and documents. Option A is wrong because hooks support async functions.