cannot do mass_assignment when inherited class ?

I got the following error when trying to create the elibrary instance

(rdb:21) @elibrary = Elibrary.new(params[:elibrary])
RuntimeError Exception: Declare either attr_protected or
attr_accessible for Elibrary, but not both.

params : "elibrary"=>{"title"=>"bingo three",
"short_description"=>"something new", "description"=>"<p>fsdfgdsg</

"}

Elibrary is declared as inherited from MediaItem, and attr are
declared accessible in MediaItem....

class MediaItem < ActiveRecord::Base
  belongs_to :media_container
  # Behaviours
  acts_as_nested_set
   attr_accessible :title, :short_description, :description
...

class Elibrary < MediaItem
end

when @elibrary = Elibrary.new, then assigning attributes to @elibrary,
save is OK.....

1- even duplicatiing the attr_accessible in both classes doesn't help
2- acts_as_nested_set brought additional fields .. ant issue with it ?

The error is somewhere in your class declaration, but the message
you're getting doesn't appear until you run attributes=.
I'd look at the other plugins you're using, as somebody's calling
attr_protected on MediaItem.

--Mat Jones