title: 'Can't convert Symbol into Integer' - issue with Simple_Form and Devise
body: <p>I'm getting the following error: </p>
  
  <p>TypeError in Devise/registrations#new
  Action Controller: Exception Caught - can't convert Symbol into Integer</p>
  
  <p>I started getting this error when I added the **'ed f.association line below.  </p>
  
  <p>This is puzzling to me because I've followed both the Rails guides association documentation, as well as the simple_form documentation: </p>
  
  <h3>Simple_Form Example</h3>
  
  <pre><code>&lt;%= simple_form_for @user do |f| %&gt;
    &lt;%= f.input :name %&gt;
    &lt;%= f.association :company %&gt;
    &lt;%= f.association :roles %&gt;
    &lt;%= f.button :submit %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <h3>My Simple_Form Execution</h3>
  
  <pre><code>&lt;%= simple_form_for(resource, :as =&gt; resource_name, :url =&gt; registration_path(resource_name), html: { class: 'form-horizontal'}) do |f| %&gt;
    &lt;%= f.error_notification %&gt;
    &lt;%= f.input :email %&gt;
    &lt;%= f.input :password %&gt;
    &lt;%= f.input :password_confirmation %&gt;
    **&lt;%= f.association :competition %&gt;**
  
    &lt;div class="form-actions"&gt;
      &lt;%= f.submit "Sign up", class: "btn btn-primary" %&gt;
    &lt;/div&gt;
  &lt;% end %&gt;
  
  &lt;%= render "devise/shared/links" %&gt;
  </code></pre>
  
  <h3>These are the models in question: </h3>
  
  <pre><code>class Competition &lt; ActiveRecord::Base
    attr_accessible :name
    has_many :rounds, :judges
  end
  
  class Judge &lt; ActiveRecord::Base
    devise :database_authenticatable, :registerable,
           :recoverable, :rememberable, :trackable, :validatable
    attr_accessible :email, :password, :password_confirmation, :remember_me, :name
    belongs_to :competition
  end
  </code></pre>
  
  <h3>I've done the following migration: </h3>
  
  <pre><code>class AddAssocToJudges &lt; ActiveRecord::Migration
    def up
      change_table :judges do |t|
        t.belongs_to :competition
      end
    end
  end
  </code></pre>
  
  <h3> Here's the application trace:</h3> 
  
  <pre><code>app/models/competition.rb:4:in `&lt;class:Competition&gt;'
  app/models/competition.rb:1:in `&lt;top (required)&gt;'
  app/views/devise/registrations/new.html.erb:9:in `block in _app_views_devise_registrations_new_html_erb__3768524909032037243_70228371062560'
  app/views/devise/registrations/new.html.erb:3:in `_app_views_devise_registrations_new_html_erb__3768524909032037243_70228371062560'
  </code></pre>