Mongoose middleware (pre/post hooks) in Express - Mini Project: Build & Apply

Mongoose Middleware with Pre and Post Hooks

📖 Scenario: You are building a simple Express app that uses Mongoose to manage a list of books in a library. You want to add middleware hooks to log messages before saving a book and after deleting a book.

🎯 Goal: Create a Mongoose schema for books, add pre-save and post-delete middleware hooks, and export the model for use in your Express app.

📋 What You'll Learn

Create a Mongoose schema named bookSchema with fields title (string) and author (string).

Create a Mongoose model named Book using bookSchema.

Add a pre-save middleware hook on bookSchema that logs "Saving book: [title]" before saving.

Add a post-delete middleware hook on bookSchema that logs "Deleted book: [title]" after deleting.

💡 Why This Matters

🌍 Real World

Middleware hooks in Mongoose are useful to add logging, validation, or other side effects automatically when data changes in a database.

💼 Career

Understanding Mongoose middleware is important for backend developers working with Node.js and MongoDB to maintain clean and maintainable data logic.

1

Create the Book Schema

Create a Mongoose schema called bookSchema with two string fields: title and author.

Express

const mongoose = require('mongoose');

// Create bookSchema with title and author fields
// Your code here

Hint

Use new mongoose.Schema({ title: String, author: String }) to create the schema.

2

Create the Book Model

Create a Mongoose model called Book using the bookSchema.

Express

const mongoose = require('mongoose');

const bookSchema = new mongoose.Schema({
  title: String,
  author: String
});

// Create Book model from bookSchema
// Your code here

Hint

Use mongoose.model('Book', bookSchema) to create the model.

3

Add Pre-save Middleware Hook

Add a pre-save middleware hook on bookSchema that logs "Saving book: [title]" before saving. Use function(next) and this.title inside the hook.

Express

const mongoose = require('mongoose');

const bookSchema = new mongoose.Schema({
  title: String,
  author: String
});

// Add pre-save hook to bookSchema
// Your code here

const Book = mongoose.model('Book', bookSchema);

Hint

Use bookSchema.pre('save', function(next) { ... }) and call next() after logging.

4

Add Post-delete Middleware Hook

Add a post middleware hook on bookSchema for the 'deleteOne' event that logs "Deleted book: [title]" after deleting. Use function() and access this.title inside the hook.

Express

const mongoose = require('mongoose');

const bookSchema = new mongoose.Schema({
  title: String,
  author: String
});

bookSchema.pre('save', function(next) {
  console.log(`Saving book: ${this.title}`);
  next();
});

// Add post-deleteOne hook to bookSchema
// Your code here

const Book = mongoose.model('Book', bookSchema);

Hint

Use bookSchema.post('deleteOne', { document: true, query: false }, function() { ... }) to access the deleted document.

Practice

1. What is the main purpose of pre middleware in Mongoose?

easy

A. To connect to the MongoDB database

B. To run code after a database operation completes

C. To define the schema structure

D. To run code before a database operation like save or remove

Solution

1. Step 1: Understand middleware timing

   Pre middleware runs before a database action, allowing preparation or validation.

2. Step 2: Differentiate pre and post hooks

   Post middleware runs after the action, so pre is for before actions.

3. Final Answer:

   To run code before a database operation like save or remove -> Option D

4. Quick Check:

   Pre middleware = before action [OK]

Hint: Pre means before the action starts [OK]

Common Mistakes:

• Confusing pre with post middleware
• Thinking pre defines schema structure
• Assuming pre connects to database

2. Which of the following is the correct syntax to add a pre-save hook in Mongoose?

easy

A. schema.on('save', function(next) { /* code */ next(); });

B. schema.pre('save', function(next) { /* code */ next(); });

C. schema.before('save', function() { /* code */ });

D. schema.post('save', function(next) { /* code */ next(); });

Solution

1. Step 1: Recall Mongoose middleware method names

   Mongoose uses pre and post methods for middleware, not before or on.

2. Step 2: Check syntax for pre-save hook

   The correct syntax is schema.pre('save', function(next) { ... next(); }); to run code before saving.

3. Final Answer:

   schema.pre('save', function(next) { /* code */ next(); }); -> Option B

4. Quick Check:

   Use schema.pre for pre hooks [OK]

Hint: Use schema.pre('event', fn) for pre hooks [OK]

Common Mistakes:

• Using schema.post instead of schema.pre for pre hooks
• Using non-existent methods like before or on
• Forgetting to call next() in middleware

3. Given this Mongoose pre-save middleware, what will be the value of doc.updatedAt after saving?

schema.pre('save', function(next) {
  this.updatedAt = new Date();
  next();
});

medium

A. Undefined because updatedAt is not set in schema

B. The date when the document was created

C. The current date and time when save is called

D. An error because next() is missing

Solution

1. Step 1: Understand pre-save middleware effect

   The middleware sets this.updatedAt to the current date before saving.

2. Step 2: Confirm middleware runs before save

   Since it runs before save, the document's updatedAt will be updated to the current time.

3. Final Answer:

   The current date and time when save is called -> Option C

4. Quick Check:

   Pre-save sets updatedAt = now [OK]

Hint: Pre-save runs before saving, so updatedAt is current time [OK]

Common Mistakes:

• Assuming updatedAt is undefined without schema field
• Confusing createdAt with updatedAt
• Thinking next() is missing causing error

4. What is wrong with this Mongoose middleware code?

schema.pre('remove', (next) => {
  console.log('Removing', this._id);
  next();
});

medium

A. Arrow function does not bind 'this', so 'this' is undefined inside middleware

B. Missing call to next() to continue middleware chain

C. Using 'remove' event is not supported in Mongoose

D. Middleware must be post, not pre, for remove

Solution

1. Step 1: Check function type in middleware

   Mongoose middleware requires normal functions to bind this to the document.

2. Step 2: Identify arrow function issue

   Arrow functions do not bind this, so this will be undefined inside the middleware.

3. Final Answer:

   Arrow function does not bind 'this', so 'this' is undefined inside middleware -> Option A

4. Quick Check:

   Use normal functions for middleware to access this [OK]

Hint: Use function() not arrow to access this in middleware [OK]

Common Mistakes:

• Using arrow functions in middleware
• Forgetting to call next() in async middleware
• Thinking remove event is unsupported

5. You want to log a message after a document is saved and also update a cache. Which Mongoose middleware setup is correct?

hard

A. Use schema.post('save', function(doc) { console.log('Saved:', this._id); updateCache(this); });

B. Use schema.pre('save', function(doc) { console.log('Saved:', this._id); updateCache(this); });

C. Use schema.post('save', (doc) => { console.log('Saved:', this._id); updateCache(this); });

D. Use schema.pre('save', (doc) => { console.log('Saved:', this._id); updateCache(this); });

Solution

1. Step 1: Identify when to run logging and cache update

   Logging and cache update should happen after saving, so use post middleware.

2. Step 2: Choose correct function syntax

   Post middleware receives the saved document as first argument; use normal function to access this if needed.

3. Final Answer:

   Use schema.post('save', function(doc) { console.log('Saved:', this._id); updateCache(this); }); -> Option A

4. Quick Check:

   Post-save + normal function for logging/cache [OK]

Hint: Use post-save with normal function for after-save tasks [OK]

Common Mistakes:

• Using pre instead of post for after-save tasks
• Using arrow functions losing this context
• Not passing doc argument in post middleware