Many-to-many with has_many through in Ruby on Rails - Performance & Optimization

Performance: Many-to-many with has_many through

MEDIUM IMPACT

This pattern affects database query performance and page load speed by controlling how many queries and joins are executed to fetch related data.

Fetching associated records in a many-to-many relationship

Ruby on Rails

class Author < ApplicationRecord
  has_many :books_authors
  has_many :books, through: :books_authors
end

# In controller or view
@authors = Author.includes(:books).all

@authors.each do |author|
  author.books.each do |book|
    puts book.title
  end
end

Using includes preloads books in a single query, avoiding N+1 queries.

📈 Performance GainReduces queries from N+1 to 2, significantly improving load speed for many authors.

Fetching associated records in a many-to-many relationship

Ruby on Rails

class Author < ApplicationRecord
  has_many :books_authors
  has_many :books, through: :books_authors
end

# In controller or view
@authors.each do |author|
  author.books.each do |book|
    puts book.title
  end
end

This triggers N+1 queries: one query for authors plus one query per author to fetch books.

📉 Performance CostTriggers N+1 database queries, increasing page load time linearly with number of authors.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
N+1 queries without includes	Minimal DOM nodes	1 reflow	Low paint cost	[X] Bad
Eager loading with includes	Minimal DOM nodes	1 reflow	Low paint cost	[OK] Good

Rendering Pipeline

The pattern affects the data fetching stage before rendering. Inefficient queries cause delays in data availability, delaying the browser's first paint.

→Data Fetching

→Rendering

⚠️ BottleneckDatabase query execution and data retrieval

Core Web Vital Affected

LCP

This pattern affects database query performance and page load speed by controlling how many queries and joins are executed to fetch related data.

Optimization Tips

1Avoid N+1 queries by using eager loading with includes.

2Preload all necessary associations before rendering views.

3Monitor database queries to keep page load times low.

Performance Quiz - 3 Questions

What is the main performance issue with using has_many through without eager loading?

A It blocks CSS from loading.

B It increases DOM nodes unnecessarily.

C It causes N+1 database queries, slowing page load.

D It causes layout shifts on the page.

DevTools: Network

How to check: Open DevTools, go to Network tab, reload the page, and filter by XHR or fetch requests to see how many database queries are triggered.

What to look for: Look for multiple repeated API calls or database queries indicating N+1 problem; fewer grouped calls indicate good performance.

Practice

1. What is the main purpose of using has_many :through in Rails models?

easy

A. To create a many-to-many relationship using a join model

B. To create a one-to-one relationship between two models

C. To validate data before saving to the database

D. To automatically generate database migrations

Solution

1. Step 1: Understand relationship types in Rails

   Rails uses associations to connect models. has_many :through is specifically for many-to-many relationships.

2. Step 2: Identify the role of join models

   It uses a join model to connect two other models, allowing extra data on the connection.

3. Final Answer:

   To create a many-to-many relationship using a join model -> Option A

4. Quick Check:

   has_many :through = many-to-many with join model [OK]

Hint: Think: many-to-many needs a join model [OK]

Common Mistakes:

• Confusing has_many :through with has_one
• Thinking it automatically creates migrations
• Using it for one-to-one relationships

2. Which of the following is the correct way to declare a many-to-many association using has_many :through in the Author model?

easy

A. has_one :books, through: :authorships

B. has_many :books, through: :authorships

C. belongs_to :books, through: :authorships

D. has_many :authorships, through: :books

Solution

1. Step 1: Identify the correct association syntax

   The model that wants to access the other through a join model uses has_many :other_model, through: :join_model.

2. Step 2: Apply to Author model

   Author has many books through authorships, so has_many :books, through: :authorships is correct.

3. Final Answer:

   has_many :books, through: :authorships -> Option B

4. Quick Check:

   has_many :books, through: :authorships = correct syntax [OK]

Hint: Remember: has_many :target, through: :join_model [OK]

Common Mistakes:

• Reversing the join and target models
• Using belongs_to instead of has_many
• Using has_one for many-to-many

3. Given these models:

class Author < ApplicationRecord
  has_many :authorships
  has_many :books, through: :authorships
end

class Authorship < ApplicationRecord
  belongs_to :author
  belongs_to :book
end

class Book < ApplicationRecord
  has_many :authorships
  has_many :authors, through: :authorships
end

What will Author.first.books.count return if the first author has 3 books linked through authorships?

medium

A. 1

B. 0

C. 3

D. An error because of missing join table

Solution

1. Step 1: Understand the association setup

   Author has many books through authorships, so calling Author.first.books fetches all books linked via authorships.

2. Step 2: Count the linked books

   If the first author has 3 books linked, Author.first.books.count returns 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Author.first.books.count = number of linked books [OK]

Hint: Count returns linked records through join model [OK]

Common Mistakes:

• Assuming count returns authorships count
• Expecting zero without checking data
• Confusing belongs_to with has_many effects

4. You have these models:

class Student < ApplicationRecord
  has_many :enrollments
end

class Enrollment < ApplicationRecord
  belongs_to :student
  belongs_to :course
end

class Course < ApplicationRecord
  has_many :enrollments
  has_many :students, through: :enrollments
end

But when you run Student.first.courses, you get an error: undefined method 'courses'. What is the likely cause?

medium

A. The has_many :courses, through: :enrollments line is missing in Student model

B. The Enrollment model is missing belongs_to :course

C. The Course model should use belongs_to :student instead of has_many

D. The database table for enrollments does not exist

Solution

1. Step 1: Check Student model associations

   The error says courses method is undefined, meaning Student model lacks has_many :courses, through: :enrollments.

2. Step 2: Confirm other associations

   Enrollment and Course models are correct; missing association in Student causes the error.

3. Final Answer:

   The has_many :courses, through: :enrollments line is missing in Student model -> Option A

4. Quick Check:

   Missing has_many through causes undefined method [OK]

Hint: Check if has_many :through is declared in the model [OK]

Common Mistakes:

• Assuming belongs_to missing causes undefined method
• Thinking Course model needs belongs_to Student
• Ignoring missing database tables as cause

5. You want to track the date when a User joins a Group using a many-to-many relationship with has_many :through. Which setup correctly allows storing this extra data?

hard

A. Add joined_on column directly to users table and use has_many :groups

B. Store joined_on in a separate table unrelated to the join

C. Use has_and_belongs_to_many :groups without a join model

D. Create a join model Membership with user_id, group_id, and joined_on columns, then use has_many :groups, through: :memberships in User

Solution

1. Step 1: Understand storing extra data in many-to-many

   Extra data on the connection requires a join model with its own columns.

2. Step 2: Apply to User and Group

   Create a join model Membership with user_id, group_id, and joined_on columns. Then User uses has_many :groups, through: :memberships.

3. Final Answer:

   Create a join model Membership with user_id, group_id, and joined_on columns, then use has_many :groups, through: :memberships in User -> Option D

4. Quick Check:

   Extra data needs join model with columns [OK]

Hint: Use join model to store extra data in many-to-many [OK]

Common Mistakes:

• Adding extra columns to main models instead of join model
• Using has_and_belongs_to_many which lacks extra data support
• Storing extra data in unrelated tables