db.sql

create table authors ( id INTEGER primary key, name TEXT not null);
create table posts ( id INTEGER primary key, author_id INTEGER not null, title text not null,
content text not null );
insert into authors (name) values ('some guy');
insert into authors (name) values ('another guy');
--cheat here, I know the first id will be 1
insert into posts(author_id, title, content)
values (1, 'My first article', 'introductory text here');
insert into posts(author_id, title, content)
values (1, 'My second article', 'itermediate  text here');
insert into posts(author_id, title, content)
values (1, 'My third article', 'more text here');
insert into posts(author_id, title, content)
values (1, 'My fourth article', 'I am tired of writing articles');
--second author
insert into posts(author_id, title, content)
values (2, 'My first article', 'introductory text here');
insert into posts(author_id, title, content)
values (2, 'My second article', 'itermediate  text here');
insert into posts(author_id, title, content)
values (2, 'My third article', 'more text here');
insert into posts(author_id, title, content)
values (2, 'My fourth article', 'I am tired of writing articles');

include.rb

require 'logger'
require 'rubygems'
require 'active_record'
ActiveRecord::Base.establish_connection( :adapter => "sqlite3",
                                         :database => "posts.db")
log = ActiveRecord::Base.logger = Logger.new('log.sql');
class Author < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :author
end

log.debug { "Without includes" }
Author.all.each do |author|
  puts author.name
  author.posts.each do |post|
      puts "  #{post.title}"
  end
end

log.debug { "With includes" }
Author.includes(:posts).all.each do |author|
  puts author.name
  author.posts.each do |post|
      puts "  #{post.title}"
  end
end

log.sql

D, [2010-12-06T20:35:08.950726 #9969] DEBUG -- : Without includes
D, [2010-12-06T20:35:09.001997 #9969] DEBUG -- :   Author Load (0.3ms)  SELECT "authors".* FROM "authors"
D, [2010-12-06T20:35:09.006422 #9969] DEBUG -- :   Post Load (0.1ms)  SELECT "posts".* FROM "posts" WHERE ("posts".author_id = 1)
D, [2010-12-06T20:35:09.008461 #9969] DEBUG -- :   Post Load (0.1ms)  SELECT "posts".* FROM "posts" WHERE ("posts".author_id = 2)
D, [2010-12-06T20:35:09.008770 #9969] DEBUG -- : With includes
D, [2010-12-06T20:35:09.009076 #9969] DEBUG -- :   Author Load (0.1ms)  SELECT "authors".* FROM "authors"
D, [2010-12-06T20:35:09.009730 #9969] DEBUG -- :   Post Load (0.2ms)  SELECT "posts".* FROM "posts" WHERE ("posts".author_id IN (1,2))