gistfile1.rb

  def booking
  	session = Session.find(params[:id])
  	booking = Booking.create(:session_id => session)
  	@user.bookings << booking
  	redirect_to users_path
  end

class Session < ActiveRecord::Base

	has_many :bookings
	has_many :users, :through => :bookings

end

class Booking < ActiveRecord::Base

	belongs_to :user
	belongs_to :session
	
	validates :user_id, :presence => true
	validates :session_id, :presence => true

end

class User < ActiveRecord::Base

	has_many :bookings
	belongs_to :sessions

end


<% @sessions.each do |session| %>

	<h1>Session <%= session.id %></h2>
	<p>Activity: <%= session.activity %></p>
	<p>Slot: <%= session.slot %></p>
	
	<p><%= link_to "Make Booking", booking_url(:id => session.id) %></p>
	
	<p>Current Bookees: <%= session.users.count %></p>
	
	
<% end %>

Routes.rb

  match '/sessions/:id/booking' => 'users#booking', :as => :booking


users index.html.erb

<% @users.each do |user| %>

	<%= user.name %></br>
	<%= user.email %></br>
	
	<h1>Bookings</h1>
	<ul>
		<% user.bookings.each do |booking| %>
	
			<li>Booking number: <%= booking.id %></li></br>
			<ul>
				<li>Session: <%= booking.session.id %></li>
				<li>Activity: <%= booking.session.activity %></li>
				<li>Slot: <%= booking.session.slot %></li><br/>				
				<li>Others who booked this session:</li>
				<ul>
					<% booking.session.users.each do |user| %>
						<li><%= link_to user.name, user_path(user) %></li>
					<% end %>
				</ul>
			</ul>
		
		<% end %>
	</ul>
	
<% end %>

create_table "bookings", :force => true do |t|
t.integer "user_id"
t.integer "session_id"
t.datetime "created_at"
t.datetime "updated_at"
end

create_table "sessions", :force => true do |t|
t.string "activity"
t.integer "slot"
t.datetime "created_at"
t.datetime "updated_at"
end

create_table "users", :force => true do |t|
t.string "name"
t.string "email"
t.datetime "created_at"
t.datetime "updated_at"
end