Ruby on Rails Pluck attribute from search and search another table with this attribute

Ok, in my Ruby on Rails application I am creating a cinema application. I am trying to implement a search where a user can click on a date, e.g. today or tomorrow, and all the films that have showings for those dates will be returned.

It is basically this: http://ift.tt/1pf4Jwb where a user is able to select a different date.

I will provide my code and then explain the problem I'm having.

views/_search.html.erb:

<%= form_tag my_path do %>
    <% date = Date.today %>
    <%= hidden_field_tag :search_string, date.strftime("%F") %>

    <div class="tab-content">
      <%= submit_tag "What's on", class: "btn" %>
    </div>

    <% date = Date.today + 1 %>
    <%= hidden_field_tag :search_string, date.strftime("%F") %>
    <%= submit_tag date.strftime("%a %D"), class: "btn" %>

    <% date = Date.today + 2 %>
    <%= hidden_field_tag :search_string, date.strftime("%F") %>
    <%= submit_tag date.strftime("%a %D"), class: "btn" %>

    <% date = Date.today + 3 %>
    <%= hidden_field_tag :search_string, date.strftime("%F") %>
    <%= submit_tag date.strftime("%a %D"), class: "btn" %>

    <% date = Date.today + 4 %>
    <%= hidden_field_tag :search_string, date.strftime("%F") %>
    <%= submit_tag date.strftime("%a %D"), class: "btn" %>

    <% date = Date.today + 5 %>
    <%= hidden_field_tag :search_string, date.strftime("%F") %>
    <%= submit_tag date.strftime("%a %D"), class: "btn" %>

    <% date = Date.today + 6 %>
    <%= hidden_field_tag :search_string, date.strftime("%F") %>
    <%= submit_tag date.strftime("%a %D"), class: "btn" %>
<% end %>

routes.rb:

post 'films/search', to: 'films#search'

films_controller.rb:

def search
    @films = Film.date_search(params[:search_string])
    else if @films.empty?
        flash.now[:alert] = "There are no films showing with that date - Showing all films."
        @films = Film.all.order :title
    end
    render :action => "index"
end

film.rb:

def self.date_search(search_string)
    film = Showing.where("show_date = ?", search_string).pluck(:film_id)
    self.where("id = ?", film)
end

Associations - film.rb:

has_many :showings
belongs_to :certificate
belongs_to :category

Associations - showing.rb:

belongs_to :film
has_many :bookings
belongs_to :screen

Schema:

create_table "showings", force: :cascade do |t|
  t.date     "show_date"
  t.time     "show_time"
  t.integer  "film_id"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
  t.integer  "screen_id"
end

create_table "films", force: :cascade do |t|
   t.string   "title"
   t.string   "synopsis"
   t.string   "director"
   t.string   "cast1"
   t.string   "cast2"
   t.string   "cast3"
   t.date     "release_date"
   t.string   "warnings"
   t.datetime "created_at",     null: false
   t.datetime "updated_at",     null: false
   t.string   "image_url"
   t.string   "certificate_id"
   t.integer  "category_id"
   t.integer  "hours"
   t.integer  "minutes"
   t.string   "video_url"
 end

I get the following error:

ActiveRecord::StatementInvalid in FilmsController#search
SQLite3::SQLException: near ",": syntax error: SELECT COUNT(*) FROM "films" WHERE (id = 3,6)

I understand why I get the error but can someone please help me solve it.