photo upload inside active_admin interface - ruby-on-rails

Question

I wan't to add image upload field inside active admin interface.This is the view where I want to be able get photo upload
I tried some earlier suggestions From here
ActiveAdmin.register Product do
form :html => { :multipart=>true } do |f|
f.inputs :new_product do
f.input :name
f.input :price
f.input :category
f.input :description
f.has_many :prod_images do |p|
p.input :photo, :as => :file, :label => "Image",:hint => p.template.image_tag(p.object.photo.url(:thumb))
p.input :_destroy, :as=>:boolean, :required => false, :label => 'Remove image'
end
end
f.buttons
end
Using this example I got such error
undefined method `klass' for nil:NilClass
It says that error is from here app/views/active_admin/resource/new.html.arb where line #1 raised
but how can I access that file, because in explorer it is not showing up?
Thanks

Answer 1

Try building a prod image, like this.
f.has_many :prod_images, f.object.prod_images.build do |p|

Answer 2

I managed to get file upload field with this code
ActiveAdmin.register Product do
form :html => { :enctype => "multipart/form-data" } do |f|
f.input :photo, :as => :file, :hint => f.template.image_tag(f.object.photo.url(:thumb))
end
But now I can't add submit buttons :D So I am still working on it :)
EDIT
ActiveAdmin.register Product do
form :html => { :enctype => "multipart/form-data" } do |f|
f.input :photo, :as => :file
f.buttons
end
end
This just shows buttons like Create and Cancel, but not showing up file field, I checked formastic examples, but without success.
EDIT2
class Product < ActiveRecord::Base
attr_accessible :category_id, :description, :manufacturer_id, :name, :photo
extend FriendlyId
has_attached_file :photo,
:styles => {
:thumb=> "100x100#",
:large => "290x170",
:medium=> "120x120"}
friendly_id :name, use: [:slugged, :history]
belongs_to :manufacturer
belongs_to :category
end