Count the amount of records are associated to another record

In my Ruby on Rails application I am creating a cinema system, and on the bookings/new page I am allowing the user to choose the amount of seats they require through a drop down menu. But what I want to do is display the number of seats that are currently free in the screen, for example if a screen has 50 seats and 7 have been booked I want the system to display: "There are 43 seats available." I know I will need a method for this but am unsure about how I would implement it and how I would show this message.

It is worth noting that a seat would only be booked for one showing, so it would be free for others, which means that the method would have to be able to count the amount of seats available for that showing.

Can someone please help.

bookings/form.html.erb:

<%= form_for @booking do |f| %>
    <%= f.hidden_field :user_id %>
    <%= f.hidden_field :showing_id %>

    <%= image_tag "thor_hammer.jpg",:size => "900x250" %>
    <h1>NEW BOOKING:</h1>
    <tr>
        <td width="350px">
            <br><%= f.label :seats_quantity, 'Please Select The Amount of Seats Required:' %>
        </td>
        <td width="300px">
            <br><%= f.select :seats_quantity, '1'..'10' %><br>
        </td>
        <td width="300px">
            <div class="actions">
                 <br><%= f.submit 'Book Showing' %>
            </div>
            <br><%= render "/error_messages", :message_header => "Cannot save: ", :target => @booking %> 
        </td>
    </tr>
<% end %>

Screen.rb:

class Screen < ActiveRecord::Base
   has_many :seats
   has_many :showings
   def screens_info
       "#{name}"
   end
end

Seat.rb:

class Seat < ActiveRecord::Base
    belongs_to :screen
end

Booking.rb:

class Booking < ActiveRecord::Base
    belongs_to :user
    belongs_to :showing
end

Showing.rb:

class Showing < ActiveRecord::Base
   belongs_to :film
   has_many :bookings
   belongs_to :screen
end

Schema:

create_table "bookings", force: :cascade do |t|
    t.integer  "user_id"
    t.integer  "showing_id"
    t.integer  "seats_quantity"
end

create_table "screens", force: :cascade do |t|
   t.string   "name"
end

create_table "showings", force: :cascade do |t|
   t.date     "show_date"
   t.time     "show_time"
   t.integer  "film_id"
   t.integer  "screen_id"
end

create_table "seats", force: :cascade do |t|
   t.string   "row_letter"
   t.integer  "row_number"
   t.integer  "screen_id"
end

It is worth noting that whilst the seats table contains the attributes row_letter and row_number a user IS NOT booking a specific seat, just the quantity of seats they require.