rails accept nested atributes for a polymorphic association - ruby-on-rails

Question

I have an error "Can't mass-assign protected attributes: Upload", but I have assigned it to be accessible.
This is a nested form with a polymorphic association.
Models
class Upload < ActiveRecord::Base
attr_accessible :link, :post_id
belongs_to :uploadable, polymorphic: true
end
class Post < ActiveRecord::Base
attr_accessible :description, :title, :uploads_attributes
has_many :uploads, as: :uploadable
accepts_nested_attributes_for :uploads, :reject_if => lambda { |a| a[:content].blank?
}, :allow_destroy => true
end
I tried too put accept_nested ... for :uploadable but tells me dont exist the association
The action new on the controller is this one
def new
#post = Post.new
#post.uploads.new
end
and here is the form for create
<%= form_for [:admin,#post], remote: true, :html => {:multipart => true} do |f| %>
<div class="field">
<%= f.label :title %><br />
<%= f.text_field :title%>
</div>
<div class="field">
<%= f.label :description%><br />
<%= f.text_area :description %>
</div>
<div>
<%= f.fields_for :upload do |builder| %>
<%= render 'upload_fields', f: builder %>
<% end %>
<%= link_to_add_fields "Add Upload", f, :uploads %>
</div>
<div class="actions">
<%= f.submit%>
</div>
<% end %>
The partial ...
<fieldset>
<%= f.label :file %><br />
<%= f.file_field :file %>
<%= f.hidden_field :_destroy %>
<%= link_to "remove", '#', class: "remove_fields" %>
</fieldset>
Dont think the javascript affects, so Im not going to put it here.
How I cna solve "Can't mass-assign protected attributes" on polymorphic asociations ?
Plz need help on this anyone. Cant belive I cant upload files, looks so simple on tutorials, and Its not working, or I get a Can't mass assign orthe upload its not saved ....

Answer 1

Try to use #post.uploads.build instead of #post.uploads.new

Answer 2

The associated model needs to know the identity of her parent to save the relationship.
I recommend you the following railscast: Polymorphic Association.
#uploads_controller.rb
before_filter :load_uploadable
def create
#upload = #uploadable.uploads.new(params[:upload])
....
end
private
def load_uploadable
resource, id = request.path.split('/')[1, 2] # /posts/1
#uploadable = resource.singularize.classify.constantize.find(id)
end

Answer 3

This line inside your view:
<%= f.fields_for :upload do |builder| %>
Should be this:
<%= f.fields_for :uploadable do |builder| %>
Because the association on the Post model is called "uploadable", not "upload".
For nested attributes to work, you will need to specify the model does accept nested attributes for this model, which can be done by putting this line underneath the belongs_to in your model:
accepts_nested_attributes_for :uploadable
And then you will need to make these attributes accessible, which you can do with this:
attr_accessible :uploadable_attributes