Why do I get the error `undefined method 'map'`?

In my Ruby on Rails application I am trying to display a three drop down menus in the _form.html.erb which are rendered from the file _booking_lookup.html.erb and get there data from the drop down menu methods in the models.

_form.html.erb:

<%= render(:partial => '/booking_lookup', :locals=> {:film => @film = Film.all, :showings => @showings = Showing.all, :seats => @seats = Seat.all, :my_path => '/films/booking_lookup' }) %>

_booking_lookup.html.erb:

<%= form_tag my_path, :method=>'post', :multipart => true do %>
<%= select_tag ('title_id'), 
    options_from_collection_for_select(@films, :id, :title_info, 0 ),
    :prompt => "Film" %>

<%= select_tag ('showings_id'), 
    options_from_collection_for_select(@showings, :id, :showing_times, 0 ),
    :prompt => "Showings" %> 

<%= select_tag ('seat_id'), 
    options_from_collection_for_select(@seats, :id, :seats_available, 0 ),
    :prompt => "Seats" %> 

<%= submit_tag 'Search' %>

film.rb:

class Film < ActiveRecord::Base

has_many :showings
belongs_to :certificate
belongs_to :category

def title_info
     "#{title}"
end
end

seat.rb:

class Seat < ActiveRecord::Base
belongs_to :screen
has_many :bookings

def seats_available
    "#{row_letter}#{row_number}"
end
end

showing.rb:

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

    def showing_times
        "#{show_date.strftime("%e %b %Y")} @ #{show_time.strftime("%H:%M")}"
    end
end

But for some reason with the line: <%= select_tag ('title_id'), options_from_collection_for_select(@films, :id, :title_info, 0 ), :prompt => "Film" %> I get the error:

NoMethodError in Bookings#new 
undefined method `map' for nil:NilClass

The weird part is that I am using a lot of this code else where, I have a _multi_search.html.erb form:

 <%= form_tag my_path, :method=>'post', :multipart => true do %>
 <!--  Genre: -->
 Search By:
 <%= select_tag ('cat_id'), 
    options_from_collection_for_select(@categories, :id, :category_info, 0 ),
    :prompt => "Genre" %>

<%= select_tag ('cert_id'), 
    options_from_collection_for_select(@certificates, :id, :certificate_info, 0 ),
    :prompt => "Age Rating" %> 

<%= text_field_tag :search_string, nil, placeholder: "ACTOR" %>
or
<%= select_tag ('title_id'), 
    options_from_collection_for_select(@films, :id, :title_info, 0 ),
    :prompt => "Film" %>
<%= submit_tag 'Search' %>
<% end %>

And is used in the application.html.erb:

<%= render(:partial => '/multi_search', :locals=> {:categories => @categories = existing_genres, :certificates => @certificates = Certificate.all, :films => @films = Film.all, :my_path => '/films/multi_find' }) %>

And that works fine.

What am I doing wrong?