_form.html.haml

%tr
  %td
    = t("filename")
    \:
  %td= f.file_field :attachment
  %td
    = t("tags")
    \:
  %td= f.autocomplete_field :tag_name, autocomplete_tag_name_images_path
- if @product.has_variants?
  %tr
    %td
      = Variant.model_name.human
      \:
    %td= f.select :viewable_id, @variants
- else
  = hidden_field_tag :product_id, @product.id
%tr
  %td
    = t("alt_text")
    \:
  %td= f.text_area :alt

edit.html.haml

# THE Begining of the form that calls _form.html.haml above

= form_for(:image, :url => admin_product_image_url(@product), :html => { :method => :put, :multipart => true }) do |f|
  %table.basic-table
    %tr
      %td
        = t("thumbnail")
        \:
      %td= link_to(image_tag(@image.attachment.url(:mini)), @image.attachment.url(:product))
    = render :partial => "form", :locals => { :f => f }
  %p.form-buttons
    = button t("update")
    = t('or')
    = link_to t("cancel"), admin_product_images_url(@product), :id => "cancel_link"

image_decorator.rb

Image.class_eval do

  begin
    acts_as_taggable # Adds tags to a tag model created by the plugin automatically.
  rescue NameError
    puts "Run rails generate acts_as_taggable_on:migrations"
  end
  
  begin
    autocomplete :tag, :name
  rescue
    puts "Run rails generate autocomplete"
  end

end

routes.rb

# Injecting routes from a gem

Rails.application.routes.draw do
  # Add your extension routes here
  namespace :admin do
    namespace :products do
      namespace :images do
        resources :autocomplete_tag_name, :on => :collection
      end
    end
  end
  namespace :images do
    resources :autocomplete_tag_name, :on => :collection
  end
end

routes.txt

admin_products_images_autocomplete_tag_name_index GET    /admin/products/images/autocomplete_tag_name(.:format)                    {:action=>"index", :controller=>"admin/products/images/autocomplete_tag_name"}
                                                  POST   /admin/products/images/autocomplete_tag_name(.:format)                    {:action=>"create", :controller=>"admin/products/images/autocomplete_tag_name"}
  new_admin_products_images_autocomplete_tag_name GET    /admin/products/images/autocomplete_tag_name/new(.:format)                {:action=>"new", :controller=>"admin/products/images/autocomplete_tag_name"}
 edit_admin_products_images_autocomplete_tag_name GET    /admin/products/images/autocomplete_tag_name/:id/edit(.:format)           {:action=>"edit", :controller=>"admin/products/images/autocomplete_tag_name"}
      admin_products_images_autocomplete_tag_name GET    /admin/products/images/autocomplete_tag_name/:id(.:format)                {:action=>"show", :controller=>"admin/products/images/autocomplete_tag_name"}
                                                  PUT    /admin/products/images/autocomplete_tag_name/:id(.:format)                {:action=>"update", :controller=>"admin/products/images/autocomplete_tag_name"}
                                                  DELETE /admin/products/images/autocomplete_tag_name/:id(.:format)                {:action=>"destroy", :controller=>"admin/products/images/autocomplete_tag_name"}