example1.rb

> blogs = Blogs.limit(5).to_a
# SELECT * FROM blogs LIMIT 5

> blogs.each { |blog| blog.posts.to_a }
# SELECT * FROM posts WHERE blog_id IN (1,2,3,4,5)

example2.rb

> blogs = Blogs.limit(5).to_a
# SELECT * FROM blogs LIMIT 5

> blogs.each do |blog| 
    if blog.posts.empty? 
      render_empty_blog
    else
      render_posts(blog.posts)
    end
  end
# SELECT COUNT(*) FROM posts WHERE blog_id = 1
# SELECT * FROM posts WHERE blog_id IN (1,2,3,4,5)

fully_load.rb

class Blog < ActiveRecord::Base
  has_many :posts, fully_load: true
end

index1.html.erb

<h1>Recent <%= @blog.name %> Posts</h1>
<ul>
  <% @posts.each do |post| %>
    <li><%= post.title %><li>
  <% end %>
<ul>

index2.html.erb

<h1>Recent <%= @blog.name %> Posts</h1>
<ul>
  <% @posts.each do |post| %>
    <li><%= post.title %> - <%= post.author.name %><li>
  <% end %>
<ul>

models.rb

class Blog < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :blog
  belongs_to :author
end

class Author < ActiveRecord::Base
  has_many :posts
end

posts_controller1.rb

class PostsController < ApplicationController

  def index
    @blog = Blog.find(params[:blog_id])
    @posts = @blog.posts.order(published_at: :desc).limit(5)
  end

end

posts_controller2.rb

class PostsController < ApplicationController

  def index
    @blog = Blog.find(params[:blog_id])
    @posts = @blog.posts.includes(:author).order(published_at: :desc).limit(5)
  end

end

query_log1.sql

SELECT * FROM posts WHERE blog_id = 1 ORDER BY published_at DESC LIMIT 5
SELECT * FROM authors WHERE id = 1 LIMIT 1
SELECT * FROM authors WHERE id = 2 LIMIT 1
SELECT * FROM authors WHERE id = 3 LIMIT 1
SELECT * FROM authors WHERE id = 4 LIMIT 1
SELECT * FROM authors WHERE id = 5 LIMIT 1

query_log2.sql

SELECT * FROM posts WHERE blog_id = 1 ORDER BY published_at DESC LIMIT 5
SELECT * FROM authors WHERE id IN (1,2,3,4,5)