Associations (hasMany, belongsTo) in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a one-to-many association where a User has many Posts.

Express

User.[1](Post);

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

AbelongsToMany

BbelongsTo

ChasOne

DhasMany

Attempts:

3 left

💡 Hint

Common Mistakes

Using belongsTo instead of hasMany reverses the relationship.

Using hasOne limits to only one related record.

In Express with Sequelize, hasMany sets a one-to-many relationship from User to Post.

2fill in blank

medium

Complete the code to define the inverse association where a Post belongs to a User.

Express

Post.[1](User);

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

AbelongsTo

BhasOne

ChasMany

DbelongsToMany

Attempts:

3 left

💡 Hint

Common Mistakes

Using hasMany here would incorrectly imply a Post owns many Users.

Using hasOne is not correct for the inverse of hasMany.

The belongsTo method sets the inverse of a hasMany association, linking Post back to User.

3fill in blank

hard

Fix the error in the association code to correctly link Comment to Post.

Express

Comment.[1](Post);

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

AhasMany

BbelongsTo

ChasOne

DbelongsToMany

Attempts:

3 left

💡 Hint

Common Mistakes

Using hasMany here would imply Comment owns many Posts, which is incorrect.

Using hasOne would limit Comment to only one Post but is less clear than belongsTo.

Each Comment belongs to one Post, so belongsTo is the correct association method.

4fill in blank

hard

Fill both blanks to create a bidirectional association between Author and Book.

Express

Author.[1](Book);
Book.[2](Author);

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

AhasMany

BbelongsTo

ChasOne

DbelongsToMany

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping hasMany and belongsTo reverses the relationship.

Using belongsToMany is for many-to-many, not one-to-many.

Authors can have many Books (hasMany), and each Book belongs to one Author (belongsTo).

5fill in blank

hard

Fill all four blanks to define a many-to-many association between Student and Course using Enrollment.

Express

Student.[1](Course, { through: Enrollment });
Course.[2](Student, { through: Enrollment });
Enrollment.[3](Student);
Enrollment.[4](Course);

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

AhasMany

BbelongsTo

CbelongsToMany

DhasOne

Attempts:

3 left

💡 Hint

Common Mistakes

Using hasMany or hasOne instead of belongsToMany for many-to-many.

Not linking the join table back to the models with belongsTo.

Many-to-many associations use belongsToMany on both models with a join table. The join table belongsTo each model.

Practice

1. In Express with Sequelize, what does the hasMany association represent between two models?

easy

A. One model instance can have multiple related instances of another model.

B. One model instance belongs to exactly one instance of another model.

C. Two models have no relationship.

D. Models share the same database table.

Solution

1. Step 1: Understand the meaning of hasMany

   The hasMany association means one record in a model can be linked to many records in another model.

2. Step 2: Compare with other associations

   belongsTo means one record belongs to one other record, not many.

3. Final Answer:

   One model instance can have multiple related instances of another model. -> Option A

4. Quick Check:

   hasMany = one-to-many [OK]

Hint: hasMany means one item links to many others [OK]

Common Mistakes:

• Confusing hasMany with belongsTo
• Thinking hasMany means no relation
• Assuming hasMany means shared tables

2. Which of the following is the correct syntax to define a belongsTo association in Sequelize for a model Comment that belongs to Post?

easy

A. Comment.belongsTo(Post);

B. Post.belongsTo(Comment);

C. Comment.hasMany(Post);

D. Post.hasMany(Comment);

Solution

1. Step 1: Identify the direction of belongsTo

   The model that belongs to another calls belongsTo on itself with the other model as argument.

2. Step 2: Apply to Comment and Post

   Since Comment belongs to Post, the syntax is Comment.belongsTo(Post);.

3. Final Answer:

   Comment.belongsTo(Post); -> Option A

4. Quick Check:

   belongsTo called on child model [OK]

Hint: belongsTo called on the model that belongs [OK]

Common Mistakes:

• Reversing the models in belongsTo
• Using hasMany instead of belongsTo
• Calling belongsTo on the wrong model

3. Given the following Sequelize association code:

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

What will await author.getBooks() return if author is an Author instance?

medium

A. An error because belongsTo is missing.

B. An array of Book instances related to that author.

C. Undefined, because getBooks is not a valid method.

D. A single Book instance related to that author.

Solution

1. Step 1: Understand hasMany and belongsTo setup

   Author has many Books, and each Book belongs to one Author, so Sequelize creates methods like getBooks on Author instances.

2. Step 2: Check what getBooks returns

   getBooks returns an array of all Book instances linked to that Author.

3. Final Answer:

   An array of Book instances related to that author. -> Option B

4. Quick Check:

   hasMany creates plural get method returning array [OK]

Hint: hasMany creates getPlural() returning array [OK]

Common Mistakes:

• Expecting a single instance instead of array
• Thinking getBooks is invalid method
• Ignoring belongsTo association

4. Consider this Sequelize association code with an error:

Order.hasMany(Item);
Item.belongsTo(Order);

// Later in code
const order = await Order.findByPk(1);
const items = await order.getItem();

What is the error and how to fix it?

medium

A. Order should use belongsTo, not hasMany.

B. belongsTo should be called on Order, not Item.

C. Method getItem() is incorrect; should be getItems() because hasMany creates plural getter.

D. findByPk is not a valid Sequelize method.

Solution

1. Step 1: Identify the association methods

   Order hasMany Item means Sequelize creates getItems() method on Order instances, not getItem().

2. Step 2: Fix the method call

   Change order.getItem() to order.getItems() to correctly fetch related items.

3. Final Answer:

   Method getItem() is incorrect; should be getItems() because hasMany creates plural getter. -> Option C

4. Quick Check:

   hasMany creates plural get method [OK]

Hint: hasMany creates plural get method, use getItems() [OK]

Common Mistakes:

• Using singular get method for hasMany
• Confusing belongsTo direction
• Thinking findByPk is invalid

5. You have two models: User and Profile. Each User has one Profile, and each Profile belongs to one User. Which Sequelize associations correctly represent this, and what methods will be available on User instances?

hard

A. User.belongsTo(Profile); Profile.hasOne(User); with user.getProfile() method.

B. User.hasMany(Profile); Profile.belongsTo(User); with user.getProfiles() method.

C. Profile.hasOne(User); User.belongsTo(Profile); with user.getProfile() method.

D. User.hasOne(Profile); Profile.belongsTo(User); with user.getProfile() method.

Solution

1. Step 1: Identify one-to-one relationship

   Each User has one Profile means hasOne is used, not hasMany.

2. Step 2: Set belongsTo on Profile

   Profile belongs to User, so Profile.belongsTo(User); is correct.

3. Step 3: Check available methods

   User instances get getProfile() method for the single related Profile.

4. Final Answer:

   User.hasOne(Profile); Profile.belongsTo(User); with user.getProfile() method. -> Option D

5. Quick Check:

   One-to-one uses hasOne + belongsTo [OK]

Hint: One-to-one uses hasOne + belongsTo with singular get method [OK]

Common Mistakes:

• Using hasMany for one-to-one
• Reversing belongsTo direction
• Expecting plural get method for one-to-one