CRUD operations with Sequelize in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Sequelize create operation?

Consider this Express route using Sequelize to create a new user. What will be the JSON response after a successful creation?

Express

app.post('/users', async (req, res) => {
  try {
    const user = await User.create({ name: 'Alice', age: 30 });
    res.json(user);
  } catch (error) {
    res.status(500).json({ error: 'Failed to create user' });
  }
});

A {"name": "Alice", "age": 30}

B {"error": "Failed to create user"}

C null

D {"id": 1, "name": "Alice", "age": 30, "createdAt": "<date>", "updatedAt": "<date>"}

💡 Hint

Sequelize returns the full created instance including auto-generated fields.

Sequelize's create method returns the full instance including auto-generated fields like id, createdAt, and updatedAt. The response sends this full object as JSON.

❓ state_output

intermediate

What is the result of this Sequelize update operation?

Given this code snippet updating a user's age, what will be the value of updatedUser.age after the operation?

Express

const updatedUser = await User.update({ age: 35 }, { where: { name: 'Alice' }, returning: true, plain: true });
console.log(updatedUser.age);

A 35

B undefined

C 30

D null

💡 Hint

The returning: true option returns the updated instance.

With returning: true and plain: true, Sequelize returns an array where the second element is the updated instance. So updatedUser.age is undefined because updatedUser is an array.

🔧 Debug

advanced

Why does this Sequelize delete operation fail with an error?

This code attempts to delete a user but throws an error. What is the cause?

Express

app.delete('/users/:id', async (req, res) => {
  try {
    await User.destroy({ id: req.params.id });
    res.json({ message: 'User deleted' });
  } catch (error) {
    res.status(500).json({ error: error.message });
  }
});

A The 'where' clause is missing in the destroy method.

B The 'id' parameter is not converted to a number.

C The route method should be 'post' instead of 'delete'.

D The destroy method does not exist in Sequelize.

💡 Hint

Sequelize's destroy method requires a 'where' object to specify which records to delete.

The destroy method requires a where option to specify the condition. Passing { id: req.params.id } directly is incorrect and causes an error.

📝 Syntax

advanced

Which option correctly retrieves all users older than 25 using Sequelize?

Select the correct Sequelize query to find all users with age greater than 25.

A User.findAll({ where: { age: { [Op.gt]: 25 } } })

B User.findAll({ where: { age: { $gt: 25 } } })

C User.findAll({ where: { age: '>25' } })

D User.findAll({ age: { gt: 25 } })

💡 Hint

Sequelize uses the Op object for operators like greater than.

Sequelize requires the use of Op.gt inside the where clause to filter by greater than.

🧠 Conceptual

expert

What happens if you call Sequelize's update method without the 'where' option?

Consider this code: await User.update({ age: 40 }); What will be the result?

A Sequelize throws a runtime error about missing 'where' clause.

B No records will be updated because 'where' is missing.

C All user records in the database will have their age set to 40.

D Only the first user record will be updated.

💡 Hint

Without a 'where' clause, Sequelize updates all rows by default.

If you omit the where option, Sequelize applies the update to all rows in the table.

Practice

1. Which Sequelize method is used to add a new record to the database?

easy

A. update()

B. findAll()

C. create()

D. destroy()

Solution

1. Step 1: Understand CRUD operations

   CRUD stands for Create, Read, Update, Delete. Each operation has a matching Sequelize method.

2. Step 2: Match method to Create operation

   The create() method is used to add new records to the database.

3. Final Answer:

   create() -> Option C

4. Quick Check:

   Create = create() [OK]

Hint: Create means add new, so use create() method [OK]

Common Mistakes:

• Confusing create() with findAll() which reads data
• Using update() to add new records
• Using destroy() which deletes records

2. Which of the following is the correct syntax to update a user's name to 'Alice' where id is 5 using Sequelize?

easy

A. User.update({ name: 'Alice' }, { where: { id: 5 } })

B. User.update({ where: { id: 5 } }, { name: 'Alice' })

C. User.update('Alice', { id: 5 })

D. User.update({ id: 5 }, { name: 'Alice' })

Solution

1. Step 1: Recall Sequelize update syntax

   The update method takes two arguments: the new values object, and the options object with a where clause.

2. Step 2: Match correct argument order and structure

   User.update({ name: 'Alice' }, { where: { id: 5 } }) correctly places the new values first and the where condition second.

3. Final Answer:

   User.update({ name: 'Alice' }, { where: { id: 5 } }) -> Option A

4. Quick Check:

   update(values, { where }) = User.update({ name: 'Alice' }, { where: { id: 5 } }) [OK]

Hint: Update needs values first, then where condition [OK]

Common Mistakes:

• Swapping the order of arguments
• Passing values inside where instead of separate object
• Using strings instead of objects for values

3. Given the code:

const users = await User.findAll({ where: { age: { [Op.gt]: 18 } } });
console.log(users.length);

What will console.log(users.length) output?

medium

A. The total number of users in the database

B. An error because Op.gt is not defined

C. Always 0 because findAll returns undefined

D. The number of users older than 18

Solution

1. Step 1: Understand findAll with where clause

   The findAll method returns all records matching the where condition. Here, it filters users with age greater than 18.

2. Step 2: Determine what users.length represents

   users is an array of matching records, so users.length is the count of users older than 18.

3. Final Answer:

   The number of users older than 18 -> Option D

4. Quick Check:

   findAll with condition returns matching array length [OK]

Hint: findAll returns array; length counts matching records [OK]

Common Mistakes:

• Thinking findAll returns undefined or null
• Ignoring the where condition filtering
• Assuming Op.gt is undefined without importing

4. What is wrong with this code snippet for deleting a user by id?

await User.destroy(id);

medium

A. destroy requires an object with a where clause, not just id

B. destroy cannot be awaited

C. destroy only works with arrays, not single values

D. destroy needs a callback function

Solution

1. Step 1: Recall destroy method signature

   Sequelize's destroy method expects an options object with a where property to specify which records to delete.

2. Step 2: Identify incorrect argument usage

   Passing just the id directly is incorrect; it must be inside { where: { id: id } }.

3. Final Answer:

   destroy requires an object with a where clause, not just id -> Option A

4. Quick Check:

   destroy({ where: { id } }) is correct [OK]

Hint: destroy needs where inside an object, not just id [OK]

Common Mistakes:

• Passing id directly instead of inside where
• Not awaiting asynchronous destroy call
• Expecting destroy to accept callback

5. You want to update multiple users' status to 'active' where their last login was before 2023-01-01. Which Sequelize code correctly does this?

hard

A. User.update({ where: { lastLogin: { [Op.lt]: '2023-01-01' } } }, { status: 'active' })

B. User.update({ status: 'active' }, { where: { lastLogin: { [Op.lt]: new Date('2023-01-01') } } })

C. User.update('active', { lastLogin: { [Op.lt]: new Date('2023-01-01') } })

D. User.update({ status: 'active' }, { lastLogin: { [Op.lt]: new Date('2023-01-01') } })

Solution

1. Step 1: Understand update method parameters

   The first argument is the values to update, the second is an options object with a where clause to filter records.

2. Step 2: Check correct use of Op.lt and date object

   User.update({ status: 'active' }, { where: { lastLogin: { [Op.lt]: new Date('2023-01-01') } } }) correctly uses [Op.lt] with a Date object inside the where clause.

3. Step 3: Verify argument order and structure

   The other options have incorrect argument order or missing where wrapper.

4. Final Answer:

   User.update({ status: 'active' }, { where: { lastLogin: { [Op.lt]: new Date('2023-01-01') } } }) -> Option B

5. Quick Check:

   update(values, { where: condition }) with Op.lt date [OK]

Hint: Update needs values first, then where with Op.lt date [OK]

Common Mistakes:

• Swapping values and where arguments
• Using string instead of Date object for date comparison
• Omitting where wrapper around condition