Associations (hasMany, belongsTo) in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding hasMany association behavior

Given the following Sequelize models, what will be the output of console.log(user.posts.length) after fetching the user with posts?

Express

const User = sequelize.define('User', { name: DataTypes.STRING });
const Post = sequelize.define('Post', { title: DataTypes.STRING });
User.hasMany(Post);
Post.belongsTo(User);

async function test() {
  const user = await User.findOne({ where: { id: 1 }, include: [Post] });
  console.log(user.posts.length);
}

A Always 0, because posts are not loaded automatically

B The number of posts related to the user (e.g., 3 if user has 3 posts)

C Undefined, because posts is not a property on user

D Throws a runtime error because include is not valid here

💡 Hint

Remember that hasMany creates an array property on the source model instance when included.

The hasMany association adds a plural property (posts) to the User instance. When you include Post in the query, Sequelize loads all related posts into that array. So user.posts.length gives the count of posts.

❓ state_output

intermediate

belongsTo association foreign key value

Consider these Sequelize models and code snippet. What will be the value of post.userId after creating a post linked to a user?

Express

const User = sequelize.define('User', { name: DataTypes.STRING });
const Post = sequelize.define('Post', { title: DataTypes.STRING });
Post.belongsTo(User);
User.hasMany(Post);

async function test() {
  const user = await User.create({ name: 'Alice' });
  const post = await Post.create({ title: 'Hello', userId: user.id });
  console.log(post.userId);
}

A The id of the user (e.g., 1)

B Undefined, because userId is not set automatically

C Null, because belongsTo does not set foreign keys

D Throws an error because userId is not a defined field

💡 Hint

Check how belongsTo creates a foreign key field on the source model.

The belongsTo association adds a foreign key field (userId) to the Post model. When creating a post with userId set to the user's id, the value is stored and accessible.

📝 Syntax

advanced

Correct association declaration syntax

Which option correctly declares a hasMany and belongsTo association between Author and Book models in Sequelize?

Express

const Author = sequelize.define('Author', { name: DataTypes.STRING });
const Book = sequelize.define('Book', { title: DataTypes.STRING });

A Author.hasMany(Book); Book.belongsTo(Author);

B Author.belongsTo(Book); Book.hasMany(Author);

C Author.hasMany(Book, { foreignKey: 'authorId' }); Book.belongsTo(Author, { foreignKey: 'authorId' });

D Author.hasMany(Book, { foreignKey: 'bookId' }); Book.belongsTo(Author, { foreignKey: 'authorId' });

💡 Hint

Check that foreign keys match on both sides of the association.

Option C correctly sets the foreign key authorId on Book for both hasMany and belongsTo. Option C misses explicit foreign keys but is valid if defaults are used. Option C reverses the relationship. Option C mismatches foreign keys.

🔧 Debug

advanced

Debugging missing association data in query

Given these models and query, why does user.posts return undefined?

Express

const User = sequelize.define('User', { name: DataTypes.STRING });
const Post = sequelize.define('Post', { title: DataTypes.STRING });
User.hasMany(Post);
Post.belongsTo(User);

async function test() {
  const user = await User.findOne({ where: { id: 1 } });
  console.log(user.posts);
}

A Because the query did not include the Post model, so posts are not loaded

B Because hasMany association was not declared correctly

C Because user.posts is always undefined in Sequelize

D Because the user with id 1 does not exist

💡 Hint

Think about how Sequelize loads associated data.

Sequelize only loads associated data if you specify it in the query using include. Without include: Post, the posts property is not set and remains undefined.

🧠 Conceptual

expert

Understanding cascading deletes with associations

In Sequelize, if you want deleting a User to automatically delete all their Posts, which association option should you use?

A Post.belongsTo(User, { onUpdate: 'RESTRICT' });

B User.hasMany(Post, { onUpdate: 'CASCADE' });

C Post.belongsTo(User, { onDelete: 'SET NULL' });

D User.hasMany(Post, { onDelete: 'CASCADE' });

💡 Hint

Think about which side controls deletion behavior and what 'CASCADE' means.

The onDelete: 'CASCADE' option on hasMany tells Sequelize to delete all related posts when the user is deleted. Other options control update behavior or set foreign keys to null, not cascading deletes.