Performance Optimization for Rails 8

Overview

Performance optimization focuses on:

• N+1 query detection and prevention
• Query optimization with eager loading
• Database indexing
• Memory management
• Batch processing

Quick Start

# Gemfile
group :development, :test do
  gem "bullet"             # N+1 detection
  gem "rack-mini-profiler" # Request profiling
end

Bullet Configuration

# config/environments/development.rb
config.after_initialize do
  Bullet.enable = true
  Bullet.alert = true
  Bullet.bullet_logger = true
  Bullet.console = true
  Bullet.rails_logger = true
end

# config/environments/test.rb
config.after_initialize do
  Bullet.enable = true
  Bullet.raise = true  # Fail tests on N+1
end

N+1 Query Problems

The Problem

# BAD: N+1 - 1 query for events, N queries for venues
@events = Event.all
@events.each { |e| e.venue.name }  # Query per event!

The Solution

# GOOD: 2 queries total
@events = Event.includes(:venue)
@events.each { |e| e.venue.name }  # No additional query

Eager Loading Methods

Method	Use When
includes	Most cases (Rails chooses strategy)
preload	Force separate queries, large datasets
eager_load	Filtering on association, need LEFT JOIN
joins	Only filtering, don't need association data

# Single association
Event.includes(:venue)

# Multiple
Event.includes(:venue, :organizer)

# Nested
Event.includes(venue: :address)

# Deep nesting
Event.includes(
  :venue, :organizer,
  vendors: [:category, :reviews],
  comments: :user
)

Query Optimization Patterns

Pattern 1: Scoped Eager Loading

class Event < ApplicationRecord
  scope :with_details, -> {
    includes(:venue, :organizer, vendors: :category)
  }
end

# Controller
@events = Event.with_details.where(account: current_account)

Pattern 2: Counter Caches

# Migration
add_column :events, :comments_count, :integer, default: 0, null: false

# Model
class Comment < ApplicationRecord
  belongs_to :event, counter_cache: true
end

# Usage (no query)
event.comments_count

Pattern 3: Select Only Needed Columns

# BAD
User.all.map(&:name)

# GOOD
User.pluck(:name)

# For objects with limited columns
User.select(:id, :name, :email)

Pattern 4: Batch Processing

# BAD: Loads all records
Event.all.each { |e| process(e) }

# GOOD: Processes in batches
Event.find_each(batch_size: 500) { |e| process(e) }

# For updates
Event.in_batches(of: 1000) do |batch|
  batch.update_all(status: :archived)
end

Pattern 5: Exists? vs Present?

# BAD: Loads all records
if Event.where(status: :active).any?
if Event.where(status: :active).present?

# GOOD: SELECT 1 LIMIT 1
if Event.where(status: :active).exists?

Pattern 6: Size vs Count vs Length

# count: Always queries database
# size: Uses counter cache if available, else count
# length: Loads collection if not loaded

# Use size (handles both cases)
events.size

Database Indexing

When to Add Indexes

Add Index For	Example
Foreign keys	account_id, user_id
WHERE columns	WHERE status = 'active'
ORDER BY columns	ORDER BY created_at DESC
JOIN columns	JOIN ON events.venue_id
Unique constraints	email, uuid

Index Types

add_index :events, :status                              # Single
add_index :events, [:account_id, :status]              # Composite
add_index :users, :email, unique: true                  # Unique
add_index :events, :event_date, where: "status = 0"    # Partial

Testing for Performance

N+1 Detection in Tests

# test/test_helper.rb
class ActiveSupport::TestCase
  setup do
    Bullet.start_request if Bullet.enable?
  end

  teardown do
    if Bullet.enable?
      Bullet.perform_out_of_channel_notifications if Bullet.notification?
      Bullet.end_request
    end
  end
end

Query Count Assertions

# test/support/query_counter.rb
module QueryCounter
  def count_queries(&block)
    count = 0
    counter = ->(*, _) { count += 1 }
    ActiveSupport::Notifications.subscribed(counter, "sql.active_record", &block)
    count
  end
end

# test/test_helper.rb
class ActiveSupport::TestCase
  include QueryCounter
end

# test/models/event_test.rb
require "test_helper"

class EventPerformanceTest < ActiveSupport::TestCase
  test "with_details makes minimal queries" do
    query_count = count_queries do
      Event.with_details.where(account: accounts(:one)).each do |e|
        e.venue&.name
        e.organizer&.name
      end
    end

    # events + venues + organizers = 3 queries max
    assert query_count <= 3, "Expected <= 3 queries, got #{query_count}"
  end
end

Missing Index Detection

# test/db/schema_test.rb
require "test_helper"

class SchemaPerformanceTest < ActiveSupport::TestCase
  test "all foreign keys have indexes" do
    connection = ActiveRecord::Base.connection

    connection.tables.each do |table|
      columns = connection.columns(table)
      fk_columns = columns.select { |c| c.name.end_with?("_id") }
      indexes = connection.indexes(table)

      fk_columns.each do |col|
        indexed = indexes.any? { |idx| idx.columns.include?(col.name) }
        assert indexed, "Missing index: #{table}.#{col.name}"
      end
    end
  end
end

Memory Optimization

# BAD: Builds large array
Event.all.map(&:name).join(", ")

# GOOD: Streams results
Event.pluck(:name).join(", ")

# BAD: Instantiates all AR objects
Event.all.each { |e| e.update!(processed: true) }

# GOOD: Direct SQL update in batches
Event.in_batches.update_all(processed: true)

Quick Fixes Reference

Problem	Solution
N+1 on belongs_to	includes(:association)
N+1 on has_many	includes(:association)
Slow COUNT	Add counter_cache
Loading all columns	Use select or pluck
Large dataset iteration	Use find_each
Missing index on FK	Add index on *_id columns
Slow WHERE clause	Add index on filtered column

Checklist

• Bullet enabled in development/test
• No N+1 queries in critical paths
• Foreign keys have indexes
• Counter caches for frequent counts
• Eager loading in controllers
• Batch processing for large datasets
• All tests GREEN