belongs_to relationship in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the belongs_to association represent in Rails?

It represents a connection where a model holds a reference to another model, meaning each instance belongs to one instance of another model.

beginner

In a belongs_to relationship, where is the foreign key stored?

The foreign key is stored in the model that declares belongs_to. This key points to the associated model's primary key.

beginner

How do you declare a belongs_to association in a Rails model?

Inside the model class, write belongs_to :other_model_name to set up the association.

intermediate

What happens if you try to save a model with a belongs_to association but the foreign key is missing?

By default, Rails will raise a validation error because belongs_to requires the associated object to be present unless specified otherwise.

intermediate

Can a belongs_to association be optional in Rails? How?

Yes, by adding optional: true to the belongs_to declaration, the association can be saved without the foreign key.

Where is the foreign key stored in a belongs_to association?

A In the model declaring <code>belongs_to</code>

B In the model declaring <code>has_many</code>

C In a join table

D In the database schema only

The foreign key is stored in the model that declares belongs_to, linking it to the associated model.

What does belongs_to :user mean inside a model?

A This model has many users

B This model is unrelated to users

C This model belongs to one user

D This model owns the user model

It means each instance of this model is linked to one user instance.

How do you make a belongs_to association optional?

A Add <code>optional: true</code> to the association

B Add <code>required: false</code> to the association

C Remove the foreign key column

D Use <code>has_one</code> instead

Adding optional: true allows saving the model without the associated record.

If a model belongs_to :post, what foreign key column is expected?

A <code>post_key</code>

B <code>post_id</code>

C <code>post_fk</code>

D <code>postRef</code>

Rails expects the foreign key column named post_id by convention.

What validation does Rails add by default to belongs_to associations?

A No validation by default

B Uniqueness validation

C Length validation

D Presence validation of the associated object

Rails requires the associated object to be present unless optional: true is set.

Explain how the belongs_to association works in Rails and where the foreign key is stored.

Think about which model 'owns' the reference.

Describe how to make a belongs_to association optional and why you might want to do that.

Consider cases where the related object might not exist yet.

Practice

1. What does the belongs_to association in Rails indicate about the relationship between two models?

easy

A. It creates a many-to-many relationship between two models.

B. The model with belongs_to is the parent and links to many child models.

C. It automatically creates a database table for the association.

D. The model with belongs_to is the child and links to one parent model.

Solution

1. Step 1: Understand the role of belongs_to

   The belongs_to association is used in the child model to link it to a single parent model.

2. Step 2: Identify the relationship direction

   This means the child model holds the foreign key and points to one parent, not many.

3. Final Answer:

   The model with belongs_to is the child and links to one parent model. -> Option D

4. Quick Check:

   belongs_to = child to one parent [OK]

Hint: Remember: belongs_to means child points to one parent [OK]

Common Mistakes:

• Confusing belongs_to with has_many
• Thinking belongs_to creates many-to-many
• Assuming belongs_to creates a new table

2. Which of the following is the correct syntax to declare a belongs_to association in a Rails model named Comment that belongs to a Post?

easy

A. belongs_to :post

B. belongs_to Post

C. has_many :post

D. belongs_to 'post'

Solution

1. Step 1: Recall Rails association syntax

   In Rails, associations use symbols for model names, so belongs_to :post is correct.

2. Step 2: Check other options for syntax errors

   belongs_to Post lacks symbol, has_many is wrong association, and quotes are not standard here.

3. Final Answer:

   belongs_to :post -> Option A

4. Quick Check:

   Use symbol for model name in belongs_to [OK]

Hint: Use symbol (colon) for model name in belongs_to [OK]

Common Mistakes:

• Using class name without colon
• Using has_many instead of belongs_to
• Using string instead of symbol

3. Given the following Rails models:

class Comment < ApplicationRecord
  belongs_to :post
end

class Post < ApplicationRecord
  has_many :comments
end

What will comment.post return if comment is a Comment instance linked to a Post with title "Hello"?

medium

A. The Post object with title "Hello"

B. An array of comments

C. A string "Hello"

D. nil

Solution

1. Step 1: Understand belongs_to usage

   The belongs_to :post means each comment has one post object accessible via comment.post.

2. Step 2: Identify what comment.post returns

   It returns the Post instance linked to that comment, which has the title "Hello".

3. Final Answer:

   The Post object with title "Hello" -> Option A

4. Quick Check:

   comment.post returns parent Post object [OK]

Hint: belongs_to returns the single parent object [OK]

Common Mistakes:

• Expecting an array instead of single object
• Expecting a string instead of object
• Assuming nil when association exists

4. You have this model code:

class Order < ApplicationRecord
  belongs_to :customer
end

When running order.customer, you get an error: undefined method 'customer'. What is the most likely cause?

medium

A. The Customer model is not defined.

B. The belongs_to should be has_many.

C. The customer_id column is missing in the orders table.

D. The association name should be plural: belongs_to :customers.

Solution

1. Step 1: Check database foreign key presence

   The belongs_to :customer requires a customer_id column in the orders table to work.

2. Step 2: Understand error cause

   If customer_id is missing, Rails cannot create the method customer, causing the error.

3. Final Answer:

   The customer_id column is missing in the orders table. -> Option C

4. Quick Check:

   Missing foreign key column causes undefined method error [OK]

Hint: Check foreign key column exists for belongs_to [OK]

Common Mistakes:

• Confusing belongs_to with has_many
• Assuming association name should be plural
• Ignoring missing database column

5. You want to create a Rails app where each Review belongs to a Product, and each product can have many reviews. You also want to ensure deleting a product deletes its reviews automatically. Which setup is correct?

hard

A. In Review model: belongs_to :product, dependent: :destroyIn Product model: has_many :reviews

B. In Review model: belongs_to :productIn Product model: has_many :reviews, dependent: :destroy

C. In Review model: has_many :productIn Product model: belongs_to :reviews, dependent: :destroy

D. In Review model: belongs_to :productIn Product model: has_many :reviews, dependent: :nullify

Solution

1. Step 1: Assign belongs_to in child model

   The Review model should have belongs_to :product to link each review to one product.

2. Step 2: Use has_many with dependent option in parent

   The Product model should have has_many :reviews, dependent: :destroy to delete reviews when product is deleted.

3. Final Answer:

   Review: belongs_to :product; Product: has_many :reviews, dependent: :destroy -> Option B

4. Quick Check:

   dependent: :destroy goes on has_many side [OK]

Hint: Put dependent: :destroy on has_many side to delete children [OK]

Common Mistakes:

• Putting dependent: :destroy on belongs_to side
• Mixing up has_many and belongs_to
• Using dependent: :nullify when deletion is wanted