01_step.sh

rails g scaffold Book title:string description:text
rails g scaffold Pubhouse title:string address:text
rails g model BookPubhouse book_id:integer pubhouse_id:integer
rake db:migrate

02_step.rb

# 'app/models/book.rb'
class Book < ActiveRecord::Base

  has_many :book_pubhouses, :dependent => :destroy # уничтожать вместе с основным экземпляром
  has_many :pubhouses, :through => :book_pubhouses # through указывает Rails приложению зависимость двух моделей через третью, теперь мы можем использовать Book.pubhouses напрямую

  # валидация
  validates :title, :presence => true
  validates :title, :uniqueness => true
  validates :title, :length => { :maximum => 150 }
  validates :description, :length => { :maximum => 2000 }
  
end

# 'app/models/pubhouse.rb'
class Pubhouse < ActiveRecord::Base

  has_many :book_pubhouses, :dependent => :destroy
  has_many :shops, :through => :book_pubhouses
  
  validates :title, :presence => true
  validates :title, :uniqueness => true
  validates :title, :length => { :maximum => 150 }
  validates :address, :length => { :maximum => 2000 }

end

# 'app/models/book_pubhouse.rb'
class BookPubhouse < ActiveRecord::Base

  belongs_to :book
  belongs_to :pubhouse
  
  validates :book_id, :presence => true
  validates :pubhouse_id, :presence => true
  validates :pubhouse_id, :uniqueness => {:scope => :book_id}

end

03_step_checkboxs.html.erb

<%= form_for(@book) do |f| %>
...

<div class="param">

    <%= f.label :pubhouses %>

    <span class="ul">

	<% Pubhouse.all.each do |pubhouse| %>
	  <label class="li"><%= check_box_tag :pubhouse_ids, pubhouse.id, @book.pubhouses.include?(pubhouse), :name => 'book[pubhouse_ids][]' %> <u><%= pubhouse.title %></u></label><br />
	<% end %>

    </span>

</div>

...
<% end %>

03_step_multiselect.html.erb

<%= form_for(@book) do |f| %>
...

<div class="param">

  <%= f.label :pubhouses %>

  <%= select_tag("book[pubhouse_ids][]", options_for_select(Pubhouse.all.collect { |pubhouse| [pubhouse.title, pubhouse.id] }, @book.pubhouses.collect { |pubhouse| pubhouse.id}), {:multiple=>true, :size=>5}) %>

</div>

...
<% end %>