An image illustrating the complexity of delegation in Ruby on Rails and the N+1 query problem. It shows a network of lines and nodes, with some nodes overwhelmed by excessive connections, symbolizing the inefficiency of the N+1 query issue. Warning signs and red flags are placed around these nodes, contrasting with other more streamlined parts of the network. The color scheme includes Ruby reds, grays, and cautionary yellow or orange, against a coding-inspired background.
Navigating Complexity: Visualizing the N+1 Query Challenge in Rails” — A depiction of the intricate risks in Rails delegation and the N+1 problem, highlighted with warning symbols amidst a digital network.

The Hidden Risks of Delegation and N+1 Queries in Rails

Alessio Bussolari
3 min readDec 11, 2023

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Delegation is a common pattern in Ruby on Rails, allowing one object to pass a method call to another object. However, when used without caution, delegation can inadvertently lead to performance issues, particularly N+1 query problems. This article explores the pitfalls of delegation in Rails and how it can conceal N+1 issues.

Understanding Delegation in Rails

Delegation in Rails is often used for object composition, where a method in one class calls a method on another object. Rails provides the delegate method to facilitate this pattern.

class Order < ApplicationRecord
belongs_to :customer
delegate :name, to: :customer, prefix: true
end

In this example, Order delegates the name method to the customer object, allowing you to call order.customer_name.

The N+1 Query Problem Hidden by Delegation

An N+1 query issue occurs when a piece of code executes a separate database query for each element in a collection, leading to a linear growth in the number of queries based on the number of elements. This is a common performance anti-pattern in Rails applications.

Delegation can mask N+1 problems because it abstracts away the details of how data is retrieved. When you call a delegated method, it’s not immediately apparent that it might result in a database query, especially if you are iterating over a collection of objects.

Example of a Hidden N+1 Issue

Consider the following scenario:

class Order < ApplicationRecord
belongs_to :customer
delegate :name, to: :customer, prefix: true
end

# Somewhere in the application
Order.all.each do |order|
puts order.customer_name
end

This code leads to an N+1 problem: for each order in Order.all, a separate query is made to fetch the customer to retrieve its name.

Best Practices to Avoid Hidden N+1 Issues

  1. Monitoring Tools: Employ monitoring tools like New Relic or Bullet to detect N+1 queries in development and production.
  2. Code Review and Testing: Regularly review code…

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Alessio Bussolari

Ruby on Rails programmer since 2009. Current CTO at COSMIC SRL, where I lead the team in creating innovative solutions.