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# What is N+1 queries ?

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 An N+1 query problem in Rails occurs when your application makes multiple database queries to retrieve associated data for a set of records. This can lead to performance issues, especially when dealing with large datasets.

Understanding the N+1 Query Problem

Imagine you have a Product model that has many Comment models. If you want to display all products along with their comments, you might write something like this:

# app/controllers/products_controller.rb
class ProductsController < ApplicationController
  def index
    @products = Product.all
  end
end

# app/views/products/index.html.erb
<% @products.each do |product| %>
  <p><%= product.name %></p>
  <% product.comments.each do |comment| %>
    <p><%= comment.body %></p>
  <% end %>
<% end %>
In this example, Rails will execute one query to fetch all products and then execute an additional query for each product to fetch its comments. If you have 10 products, this results in 11 queries (1 for products + 10 for comments), hence the term "N+1".

Solving the N+1 Query Problem

To solve this problem, you can use eager loading with the includes method. This loads all the associated records in a single query:

# app/controllers/products_controller.rb
class ProductsController < ApplicationController
  def index
    @products = Product.includes(:comments).all
  end
end
With this change, Rails will execute two queries: one to fetch all products and one to fetch all comments associated with those products.

Example with Eager Loading

# app/controllers/products_controller.rb
class ProductsController < ApplicationController
  def index
    @products = Product.includes(:comments).all
  end
end

# app/views/products/index.html.erb
<% @products.each do |product| %>
  <p><%= product.name %></p>
  <% product.comments.each do |comment| %>
    <p><%= comment.body %></p>
  <% end %>
<% end %>

Tools to Detect N+1 Queries
  • Bullet Gem: This gem helps to detect N+1 queries and suggests optimizations.
  • # Gemfile
gem 'bullet'
Conclusion

N+1 queries can significantly impact the performance of your Rails application. By using eager loading with the includes method, you can reduce the number of queries and improve performance.