Mongoose middleware (pre/post hooks) in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a pre-save hook modifies a document field?

Consider a Mongoose schema with a pre('save') hook that changes a field value before saving. What will be the final value stored in the database?

Express

const userSchema = new mongoose.Schema({ name: String, active: Boolean });
userSchema.pre('save', function(next) {
  this.active = true;
  next();
});
const User = mongoose.model('User', userSchema);

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

A undefined

B true

C false

D null

💡 Hint

The pre-save hook runs before the document is saved to the database and can modify fields.

The pre-save hook sets the active field to true before saving, so the saved document has active: true. The console logs the updated value.

❓ state_output

intermediate

What is the output of a post-remove hook logging the removed document?

Given a Mongoose model with a post('remove') hook that logs the removed document, what will be printed after calling remove() on a document?

Express

const itemSchema = new mongoose.Schema({ name: String });
itemSchema.post('remove', function(doc) {
  console.log(doc.name);
});
const Item = mongoose.model('Item', itemSchema);

const item = new Item({ name: 'Book' });
await item.save();
await item.remove();

A Error: remove is not a function

B undefined

C null

D "Book"

💡 Hint

The post-remove hook receives the removed document as an argument.

The post-remove hook logs the name field of the removed document, which is "Book".

📝 Syntax

advanced

Which option correctly defines a pre-update hook using async/await?

You want to run code before updating a document using updateOne. Which code snippet correctly defines an async pre-update hook?

A schema.pre('updateOne', async function(next) { await someAsyncTask(); next(); });

B schema.pre('updateOne', function(next) { await someAsyncTask(); next(); });

C schema.pre('updateOne', async function() { await someAsyncTask(); });

D schema.pre('updateOne', function() { someAsyncTask().then(() => next()); });

💡 Hint

Async hooks can return a promise or use next callback, but mixing both can cause errors.

Option C correctly uses an async function without next callback. Option C uses await inside a non-async function causing syntax error. Option C mixes async and next callback which is discouraged. Option C calls next without defining it as a parameter causing ReferenceError.

🔧 Debug

advanced

Why does this post-save hook not log the updated document?

A developer writes this post-save hook but it logs the old document state instead of the updated one. Why?

Express

schema.post('save', function(doc) {
  console.log(doc.modifiedField);
});

await Model.findByIdAndUpdate(id, { modifiedField: 'newValue' });

A findByIdAndUpdate does not trigger save middleware

B post-save hook only runs on new documents

C The hook must be pre-save to see changes

D The document passed to post-save is a copy, not updated

💡 Hint

Not all update methods trigger save middleware in Mongoose.

Mongoose's findByIdAndUpdate bypasses save middleware. Only save() triggers pre/post save hooks. To run middleware on updates, use pre/post('findOneAndUpdate') hooks.

🧠 Conceptual

expert

How does Mongoose handle errors thrown in async pre hooks?

If an async pre hook throws an error, what is the effect on the document operation?

A The operation is aborted and the error is passed to the callback or promise rejection

B The error is ignored and the operation continues normally

C The error causes the server to crash immediately

D The hook retries the operation automatically once

💡 Hint

Middleware errors usually stop the operation and propagate the error.

When an async pre hook throws an error, Mongoose aborts the operation and passes the error to the caller, either via callback or promise rejection. This prevents saving or updating invalid data.

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