Update action for Carrierwave 1.0 Multifile upload with single controller (delete several files or delete all)

_form.html.erb

<%= content_tag :div, :class => "gallery_rows" do -%>
  <% gallery.photo.each do |image| %>
    <%= content_tag :div, :class => "gallery_image", :id => "gallery_image_#{gallery.photo.index(image)+1}" do -%><%# delete by index %>
      <%= content_tag :div, :class => "imagethumb" do -%> 
        <%= image_tag(image.thumb.url) if gallery.photo? -%>
      <% end %>
      <p>Delete photo?</p>
      <div class="imageswitch">
        <%= check_box_tag 'delete_this_images[]', gallery.photo.index(image),
              false, id: "delete_image_#{gallery.photo.index(image)+1}", class: 'imageswitch-cb'
        -%>
        <%= 
            label_tag "delete_image_#{gallery.photo.index(image)+1}", nil, class: 'imageswitch_label' do
              concat content_tag(:div, '', class: 'imageswitch-inner')
              concat content_tag(:div, '', class: 'imageswitch-switch')
            end
        -%>
      </div>
    <% end %>
  <% end %>
<% end unless gallery.new_record? || !gallery.photo? %>

array_refinement.rb

module ArrayRefinement
  refine Array do
    def delete_at_multi(arr)
      arr = arr.sort.reverse!
      arr.each { |i| self.delete_at i }
      self
    end
  end
end

=begin
    from http://stackoverflow.com/a/7964648/4814907

    delete several by index items from array
    delete highest indexes first: iterate in reverse, so
      it deletes behind the iteration not ahead of it, so
      elements are not shifted
    array.delete_at is mutating the array while you are iterating.
    does not save db if arr is nil, see controller
=end

gallery_controller.rb

using ArrayRefinement
# ...
def update
  if (gallery.update_attributes gallery_params.except(:photo)) # if success - operate photos:
    if params[params[:controller].classify.underscore.to_sym][:photo].present? || params[:delete_this_images].present?
      photos = gallery.photo # getter
      # add new image to the old ones:
      photos += params[params[:controller].classify.underscore.to_sym][:photo] unless 
                params[params[:controller].classify.underscore.to_sym][:photo].nil? #no implicit conversion of nil into array
      # delete several old images(only in db, not in public folder):
      if params[:delete_this_images].present? # nilClass check
        photos.delete_at_multi(params[:delete_this_images].map(&:to_i)) #delete pictures by index got by checkbox in view
      end # see module bellow for details
      gallery.photo = photos # setter
    end
    if gallery.photo.size.zero? #if deleted all photos
      FileUtils.rm_rf("public/uploads/#{params[:controller].classify.underscore.to_sym}/photo/#{params[:id]}") #empty folder
      gallery.update_column(:photo, nil) #empty column
    end
    if gallery.save
      flash[:notice] = t(:edited) + " " + t(:success).concat("!")
      redirect_to (send("#{params[:controller].classify.underscore.to_sym}_path"))
    else
      render :edit
    end
  else
    render :edit
  end
end
# ...
private
helper_method def gallery
  @gallery ||= found_gallery || nil # ...
end
# ...
def found_gallery
  %w(show edit update destroy).
    include?(params[:action]) && params[:controller].
    classify.constantize.find(params[:id])
end
# ...
def gallery_params
  arr = [{photo: []}] # ...
  arr += [{delete_this_images: []}] if %w(edit update).include?(params[:action])
  params.require(params[:controller].classify.underscore.to_sym).permit(arr)
end