<%= article.title %>
By <%= article.author.name %>
<% end %>
```
For 100 articles, this code generates 101 SQL queries. Rails logs clearly show the problem with repetitive queries.
```sql
-- Rails logs showing the N+1 problem
-- 1 initial query
SELECT "articles".* FROM "articles"
-- N queries for authors (repeated for each article)
SELECT "users".* FROM "users" WHERE "users"."id" = 1 LIMIT 1
SELECT "users".* FROM "users" WHERE "users"."id" = 2 LIMIT 1
SELECT "users".* FROM "users" WHERE "users"."id" = 3 LIMIT 1
-- ... 97 more queries
```
## Solving with includes
The `includes` method is the most common and recommended solution for fixing N+1 problems. It tells ActiveRecord to preload associations in one or two optimized queries.
```ruby
# app/controllers/articles_controller.rb
# Solution with includes - preloading authors
class ArticlesController < ApplicationController
def index
# Preloads authors with articles
# Generates only 2 queries instead of N+1
@articles = Article.includes(:author).all
end
end
```
With `includes`, ActiveRecord executes only two queries regardless of article count. The first retrieves all articles, the second retrieves all relevant authors.
```sql
-- Rails logs with includes (only 2 queries)
SELECT "articles".* FROM "articles"
SELECT "users".* FROM "users" WHERE "users"."id" IN (1, 2, 3, 4, 5, ...)
```
Nested associations can also be preloaded using hash syntax. This approach is essential when views access multiple levels of associations.
```ruby
# app/controllers/articles_controller.rb
# Preloading nested associations
class ArticlesController < ApplicationController
def index
# Preloads author -> company and all comments
@articles = Article.includes(author: :company, comments: :user)
end
end
```
> **Golden Rule**
>
> If a view accesses an association inside a loop, that association must be preloaded in the controller with `includes`. Always verify association access patterns in views.
## Differences Between includes, preload, and eager_load
Rails provides three methods for association preloading. Each uses a different SQL strategy, with specific use cases.
### preload: Separate Queries
The `preload` method always executes separate queries for each association. It works efficiently when no WHERE conditions filter on associations.
```ruby
# app/models/article.rb
# preload always uses separate queries
class Article < ApplicationRecord
scope :with_authors, -> { preload(:author) }
end
# Usage in controller
@articles = Article.with_authors.limit(20)
# SQL generated:
# SELECT "articles".* FROM "articles" LIMIT 20
# SELECT "users".* FROM "users" WHERE "users"."id" IN (1, 2, 3, ...)
```
### eager_load: LEFT OUTER JOIN
The `eager_load` method uses a LEFT OUTER JOIN to load data in a single query. It becomes mandatory when filtering or sorting on association columns.
```ruby
# app/controllers/articles_controller.rb
# eager_load allows filtering on associations
class ArticlesController < ApplicationController
def verified_authors
# Filters articles by author status
# Requires eager_load because WHERE references users
@articles = Article.eager_load(:author)
.where(users: { verified: true })
.order("users.name ASC")
end
end
# SQL generated (single query with JOIN):
# SELECT "articles"."id", "articles"."title", ...
# FROM "articles"
# LEFT OUTER JOIN "users" ON "users"."id" = "articles"."author_id"
# WHERE "users"."verified" = TRUE
# ORDER BY "users"."name" ASC
```
### includes: Smart Behavior
The `includes` method automatically chooses the best strategy. It uses `preload` by default, but switches to `eager_load` if a WHERE clause references the association.
```ruby
# app/controllers/articles_controller.rb
# includes adapts automatically to context
class ArticlesController < ApplicationController
def index
# No condition on association: uses preload (2 queries)
@articles = Article.includes(:author).all
end
def by_verified_authors
# With condition on association: uses eager_load (JOIN)
@articles = Article.includes(:author)
.where(users: { verified: true })
end
end
```
The following table summarizes differences between the three methods.
| Method | SQL Strategy | Use Case |
|--------|--------------|----------|
| `preload` | Separate queries | Simple preloading, no filtering |
| `eager_load` | LEFT OUTER JOIN | Filtering/sorting on associations |
| `includes` | Automatic | General use, recommended default |
## Automated N+1 Detection
Manual detection of N+1 problems is tedious and error-prone. Several tools automate this detection in development and CI.
### Bullet: Real-Time Detection
The Bullet gem analyzes SQL queries in real-time and alerts on detected N+1 problems. It also suggests appropriate fixes.
```ruby
# Gemfile
# Bullet detects N+1 in development
group :development do
gem 'bullet'
end
```
Configuration in the development environment enables various alert modes.
```ruby
# config/environments/development.rb
# Bullet configuration to detect N+1
Rails.application.configure do
config.after_initialize do
Bullet.enable = true
# Display JavaScript alert in browser
Bullet.alert = true
# Add footer with details
Bullet.bullet_logger = true
# Display in Rails logs
Bullet.rails_logger = true
# Raise exception (useful in CI)
Bullet.raise = false
end
end
```
When an N+1 problem is detected, Bullet displays an explicit message with the recommended solution.
```text
# Example Bullet alert in logs
USE eager loading detected
Article => [:author]
Add to your query: .includes([:author])
Call stack:
/app/views/articles/index.html.erb:5
```
> **Bullet in CI**
>
> In continuous integration, enabling `Bullet.raise = true` causes tests to fail if an N+1 problem is detected. This prevents performance regressions.
### Prosopite: Lightweight Alternative
The Prosopite gem offers a lighter alternative to Bullet, with minimal configuration and test compatibility.
```ruby
# Gemfile
# Prosopite as an alternative to Bullet
group :development, :test do
gem 'prosopite'
end
```
```ruby
# config/environments/development.rb
# Prosopite configuration
Rails.application.configure do
config.after_initialize do
Prosopite.rails_logger = true
Prosopite.raise = Rails.env.test?
end
end
```
## Advanced Optimization Techniques
Beyond basic methods, several techniques allow fine-tuned ActiveRecord query optimization.
### Strict Loading: Default Prevention
Rails 6.1+ provides strict loading mode that raises an exception if a non-preloaded association is accessed. This preventive approach forces N+1 resolution during development.
```ruby
# app/models/article.rb
# Enable strict loading by default on the model
class Article < ApplicationRecord
# Any non-preloaded association access raises an exception
self.strict_loading_by_default = true
belongs_to :author
has_many :comments
end
```
Strict loading can also be enabled on a specific query basis.
```ruby
# app/controllers/articles_controller.rb
# Strict loading on a specific query
class ArticlesController < ApplicationController
def index
# Raises StrictLoadingViolationError if a non-included
# association is accessed
@articles = Article.strict_loading.includes(:author)
end
end
```
### Select and Pluck for Partial Data
When only certain columns are needed, `select` and `pluck` reduce the amount of data transferred from the database.
```ruby
# app/controllers/reports_controller.rb
# Optimization with select and pluck
class ReportsController < ApplicationController
def titles_only
# select returns Article objects with only id and title
@articles = Article.select(:id, :title)
end
def title_array
# pluck returns an Array of values, not AR objects
# More performant when only values are needed
@titles = Article.pluck(:title)
# => ["First article", "Second article", ...]
end
end
```
### Counter Cache for Counts
Association counts (`article.comments.count`) generate a SQL query on each call. Counter cache stores this count directly in the parent table.
```ruby
# app/models/comment.rb
# Counter cache configuration
class Comment < ApplicationRecord
# Rails automatically maintains the counter in articles.comments_count
belongs_to :article, counter_cache: true
end
```
The migration adds the count column with a default value.
```ruby
# db/migrate/20260223_add_comments_count_to_articles.rb
# Migration to add counter cache
class AddCommentsCountToArticles < ActiveRecord::Migration[7.1]
def change
add_column :articles, :comments_count, :integer, default: 0, null: false
# Initialize counters for existing data
Article.find_each do |article|
Article.reset_counters(article.id, :comments)
end
end
end
```
After this configuration, `article.comments_count` reads the column directly without additional SQL queries.
```ruby
# app/views/articles/index.html.erb
# Using counter cache (no SQL query)
<% @articles.each do |article| %>