Mongoose middleware (pre/post hooks) in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a pre-save hook that logs a message before saving a document.

Express

schema.pre('save', function(next) {
  console.log('About to save document');
  [1]();
});

Drag options to blanks, or click blank then click option'

Acallback

Bdone

Cnext

Dproceed

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to call the next function, causing the save to hang.

Calling a wrong function name like done or callback.

The next function must be called to continue the middleware chain in Mongoose pre hooks.

2fill in blank

medium

Complete the code to add a post-save hook that logs the saved document.

Express

schema.post('save', function(doc) {
  console.log('Document saved:', [1]);
});

Drag options to blanks, or click blank then click option'

Adoc

Bthis

Cdocument

DsavedDoc

Attempts:

3 left

💡 Hint

Common Mistakes

Using this instead of the passed document in post hooks.

Trying to access document properties without the argument.

In post-save hooks, the saved document is passed as the first argument, commonly named doc.

3fill in blank

hard

Fix the error in the pre-remove hook to properly call the next middleware.

Express

schema.pre('remove', function([1]) {
  console.log('Removing document');
  next();
});

Drag options to blanks, or click blank then click option'

Adone

Bnext

Ccallback

Dproceed

Attempts:

3 left

💡 Hint

Common Mistakes

Not declaring the next parameter but calling next() inside the function.

Using a different parameter name but calling next().

The middleware function must declare next as a parameter to call it properly.

4fill in blank

hard

Fill both blanks to create a pre-find hook that modifies the query to only find active users.

Express

schema.pre('[1]', function() {
  this.[2]({ active: true });
});

Drag options to blanks, or click blank then click option'

Afind

Bwhere

Cdelete

Dsave

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong hook name like 'save' or 'delete'.

Calling a method that does not exist on the query object.

The pre-find hook runs before a find query. The this context is the query, so calling this.where() modifies the query conditions.

5fill in blank

hard

Fill all three blanks to create a post-update hook that logs the update result and calls next.

Express

schema.post('[1]', function(result, [2]) {
  console.log('Update result:', [3]);
  next();
});

Drag options to blanks, or click blank then click option'

AupdateOne

Bnext

Cresult

Dsave

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong hook name like 'save'.

Swapping the order of parameters.

Not calling next() to continue middleware.

The post-update hook listens to 'updateOne'. The second parameter is the next callback. The first parameter is the update result.

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