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.