N+1 queries example

n_plus_one.rb

class User < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :user
end

# It will hit 11 times your database, that's bad!
User.limit(10).each do |user|
  user.posts.each do |post|
    puts post.title
  end
end

# It will hit 2 times your database, that's nice!
User.limit(10).preload(:posts).each do |user|
  user.posts.each do |post|
    puts post.title
  end
end