Mass assignment error with polymorphic association - ruby-on-rails

Question

I'm getting a mass assignment error when submitting a nested form for a has_one polymorphic model. The form is trying to create Employee and Picture instances based on the polymorphic association Rails guide.
I would appreciate literally ANY functioning example of a nested creation form for a has_one polymorphic model! I know there are tons of questions on mass assignment errors but I've never seen a working example with polymorphic associations.
Models
class Picture < ActiveRecord::Base
belongs_to :illustrated, :polymorphic => true
attr_accessible :filename, :illustrated
end
class Employee < ActiveRecord::Base
has_one :picture, :as => :illustrated
accepts_nested_attributes_for :picture
attr_accessible :name, :illustrated_attribute
end
Migrations
create_table :pictures do |t|
t.string :filename
t.references :illustrated, polymorphic: true
end
create_table :employees do |t|
t.string :name
end
controllers/employees_controller.rb
...
def new
#employee = Employee.new
#employee.picture = Picture.new
end
def create
#employee = Employee.new(params[:employee])
#employee.save
end
...
Error
Can't mass-assign protected attributes: illustrated
app/controllers/employees_controller.rb:44:in `create'
{"utf8"=>"✓", "authenticity_token"=>"blah"
"employee"=>{"illustrated"=>{"filename"=>"johndoe.jpg"},
"name"=>"John Doe"},
"commit"=>"Create Employee"}
In the models, I've tried every permutation of :illustrated, :picture, :illustrated_attribute, :illustrated_attributes, :picture_attribute, :picture_attributes, etc. Any tips or examples?
EDIT:
_form.html.erb
<%= form_for(#employee) do |f| %>
<%= f.fields_for :illustrated do |form| %>
<%= form.text_field :filename %>
<% end %>
<%= f.text_field :name %>
<%= f.submit %>
<% end %>

Answer 1

You need to specify the nested_attributes appropriately. accepts_nested_attributes_for takes the name of the association as parameter and then you need to add the same assoc_attributes to your attr_accessible. So change your Employee model to
class Employee < ActiveRecord::Base
has_one :picture, :as => :illustrated
accepts_nested_attributes_for :picture
attr_accessible :name, :picture_attribute
end
And change the field_for line in the view code to
<%= f.fields_for :picture do |form| %>