title: Simple_form has_and_belongs_to_many association is not updating
body: <p>I have models for Venues and Photos:</p>
  
  <pre><code>class Venue &lt; ActiveRecord::Base
    has_and_belongs_to_many :photos
  
    validates :name, presence: true
    validates :permalink, presence: true, uniqueness: true
  
    def to_param
      permalink
    end
  end
  
  class Photo &lt; ActiveRecord::Base
    mount_uploader :image, PhotoUploader
  
    has_and_belongs_to_many :venues
  end
  </code></pre>
  
  <p>I'm using simple_form to make the following form:</p>
  
  <pre><code>&lt;div class="content-box"&gt;
    &lt;%= simple_form_for [:admin, @venue] do |f| %&gt;
      &lt;%= f.input :name %&gt;
      &lt;%= f.input :permalink %&gt;
      &lt;%= f.input :description, input_html: { cols: 100, rows: 6 }%&gt;
      &lt;%= f.input :address %&gt;
      &lt;%= f.input :city %&gt;
      &lt;%= f.input :phone %&gt;
      &lt;%= f.association :photos %&gt; 
      &lt;%= f.button :submit, class: 'small radius' %&gt;
    &lt;% end %&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>And here is my controller for the Edit and Update methods:</p>
  
  <pre><code>class Admin::VenuesController &lt; ApplicationController
  
    def edit
      @venue = Venue.find_by_permalink!(params[:id])
      @photos = @venue.photos.all
    end
  
    def update
      @venue = Venue.find_by_permalink!(params[:id])
      if @venue.update_attributes(venue_params)
        redirect_to edit_admin_venue_path(@venue), notice: 'Venue was successfully updated.'
      else
        render action: "edit"
      end
    end
  
    private
  
    def venue_params
      params.require(:venue).permit(:name, :permalink, :description, :address, :city, :phone, :photo_ids)
    end
  end
  </code></pre>
  
  <p>The problem is that when I update using the form, all of the attributes for the venue model update fine, but the photo_ids are not updated or stored. I'm guessing it's something simple, but I'm not sure what it is. I'm using Rails 4, btw.</p>