Updating documents in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express route when updating a document?

Consider this Express route that updates a user document by ID. What will the response be if the user exists and the update is successful?

Express

app.put('/users/:id', async (req, res) => {
  try {
    const updatedUser = await User.findByIdAndUpdate(req.params.id, req.body, { new: true });
    if (!updatedUser) {
      return res.status(404).send('User not found');
    }
    res.json(updatedUser);
  } catch (error) {
    res.status(500).send('Server error');
  }
});

A Responds with the updated user document in JSON format.

B Responds with a 404 status and 'User not found' message regardless of user existence.

C Responds with a 500 status and 'Server error' message on successful update.

D Responds with the original user document before update.

💡 Hint

Look at the option { new: true } in findByIdAndUpdate.

The option { new: true } makes findByIdAndUpdate return the updated document. So the response sends the updated user in JSON.

📝 Syntax

intermediate

Which option correctly updates a document using Mongoose in Express?

You want to update a document by ID and return the updated document. Which code snippet is correct?

A User.update(id, update, { new: true });

B User.findByIdAndUpdate(id, update, { new: true }, callback);

C User.updateOne(id, update, { returnNew: true });

D User.findByIdAndUpdate(id, update); res.send(updatedUser);

💡 Hint

Check the Mongoose method names and options for returning updated documents.

findByIdAndUpdate with { new: true } returns the updated document. updateOne and update do not return the updated document by default.

🔧 Debug

advanced

Why does this Express update route always return the original document instead of the updated one?

Look at this code snippet. Why does it return the original document before update instead of the updated one?

Express

app.put('/items/:id', async (req, res) => {
  const item = await Item.findByIdAndUpdate(req.params.id, req.body);
  res.json(item);
});

A Because res.json cannot send updated documents.

B Because the update data in req.body is ignored by findByIdAndUpdate.

C Because the route method should be POST, not PUT.

D Because findByIdAndUpdate defaults to returning the original document unless { new: true } is set.

💡 Hint

Check the options passed to findByIdAndUpdate.

By default, findByIdAndUpdate returns the document before the update. You must set { new: true } to get the updated document.

❓ state_output

advanced

What is the value of 'result.modifiedCount' after this update operation?

Given this Mongoose update operation, what will be the value of result.modifiedCount if one document was updated successfully?

Express

const result = await User.updateOne({ _id: userId }, { $set: { name: 'Alice' } });
// What is result.modifiedCount?

A 1

B 0

C undefined

D Throws an error

💡 Hint

Check the Mongoose updateOne result object properties.

updateOne returns an object with modifiedCount indicating how many documents were modified. If one document updated, modifiedCount is 1.

🧠 Conceptual

expert

Which option best explains why using findOneAndUpdate with { runValidators: true } is important?

Why should you include { runValidators: true } when updating documents with findOneAndUpdate in Mongoose?

A It automatically creates a new document if none matches the query.

B It returns the original document instead of the updated one.

C It ensures the update respects schema validation rules before saving changes.

D It disables all validation to speed up the update process.

💡 Hint

Think about data integrity and schema rules during updates.

By default, findOneAndUpdate does not run schema validators. Setting runValidators: true enforces validation on the update.

Practice

1. What does the findByIdAndUpdate method do in Express with Mongoose?

easy

A. It updates a document by its ID and returns the updated document.

B. It deletes a document by its ID.

C. It creates a new document with a given ID.

D. It finds a document but does not update it.

Solution

1. Step 1: Understand the method purpose

   findByIdAndUpdate is used to find a document by its ID and update it with new data.

2. Step 2: Check the return value

   When used with { new: true }, it returns the updated document, not the old one.

3. Final Answer:

   It updates a document by its ID and returns the updated document. -> Option A

4. Quick Check:

   Update document by ID = A [OK]

Hint: Remember: findByIdAndUpdate updates and returns new data [OK]

Common Mistakes:

• Confusing update with delete operation
• Expecting it to create a new document
• Thinking it only finds without updating

2. Which of the following is the correct syntax to update a document by ID and return the updated document in Express with Mongoose?

easy

A. Model.findOneAndUpdate(id, update);

B. Model.updateById(id, update, callback);

C. Model.update(id, update, { returnNew: true });

D. Model.findByIdAndUpdate(id, update, { new: true }, callback);

Solution

1. Step 1: Identify correct method and parameters

   The correct method is findByIdAndUpdate with parameters: id, update object, options, and callback.

2. Step 2: Confirm option for returning updated document

   The option { new: true } ensures the updated document is returned.

3. Final Answer:

   Model.findByIdAndUpdate(id, update, { new: true }, callback); -> Option D

4. Quick Check:

   Correct syntax includes { new: true } = C [OK]

Hint: Use { new: true } option to get updated document [OK]

Common Mistakes:

• Using wrong method name like updateById
• Omitting the { new: true } option
• Passing incorrect parameters order

3. Given the code snippet:

Model.findByIdAndUpdate('123', { name: 'Alice' }, { new: true }, (err, doc) => {
  if (err) return console.error(err);
  console.log(doc.name);
});

What will be printed if the update is successful?

medium

A. The old name before update

B. Alice

C. undefined

D. An error message

Solution

1. Step 1: Understand the update and options

   The update changes the name to 'Alice' and { new: true } returns the updated document.

2. Step 2: Check the console output

   The callback logs doc.name, which will be 'Alice' after update.

3. Final Answer:

   Alice -> Option B

4. Quick Check:

   Updated document name logged = B [OK]

Hint: With { new: true }, console logs updated value [OK]

Common Mistakes:

• Expecting old value instead of updated
• Not using { new: true } option
• Misreading callback parameters

4. Identify the error in this code snippet for updating a document:

Model.findByIdAndUpdate('123', { age: 30 }, (err, doc) => {
  if (err) console.log(err);
  else console.log(doc);
});

medium

A. The update object should be a string

B. Using wrong method name, should be updateById

C. Missing the { new: true } option to get updated document

D. Callback function parameters are incorrect

Solution

1. Step 1: Check method usage

   The method findByIdAndUpdate is correct and callback parameters are valid.

2. Step 2: Identify missing option

   Without { new: true }, the returned document is the old one, not updated.

3. Final Answer:

   Missing the { new: true } option to get updated document -> Option C

4. Quick Check:

   Return updated doc requires { new: true } = D [OK]

Hint: Add { new: true } to get updated document in callback [OK]

Common Mistakes:

• Assuming updateById is a valid method
• Expecting updated doc without { new: true }
• Thinking update object must be string

5. You want to update a user's email only if the new email is not empty and valid. Which approach correctly updates the document safely in Express with Mongoose?

hard

A. Check email validity before calling findByIdAndUpdate, then update with { new: true } option.

B. Call findByIdAndUpdate directly without validation to save time.

C. Use findByIdAndUpdate with an empty update object if email is invalid.

D. Update the document without callback and ignore errors.

Solution

1. Step 1: Validate input before updating

   Always check if the new email is valid and not empty before updating to avoid bad data.

2. Step 2: Use findByIdAndUpdate with { new: true }

   Update the document with the valid email and use { new: true } to get the updated document.

3. Final Answer:

   Check email validity before calling findByIdAndUpdate, then update with { new: true } option. -> Option A

4. Quick Check:

   Validate input + update with new:true = A [OK]

Hint: Always validate input before updating documents [OK]

Common Mistakes:

• Skipping validation and risking bad data
• Updating with empty or invalid data
• Ignoring errors by not using callbacks