simple_form custom input for a composed_of aggregate attribute in Rails - ruby-on-rails-3

Question

I'm trying to write a custom input for the simple_form gem for a Rails 3 model which has a composed_of aggregate attribute. See the example below.
I tried using fields_for but it generates params like "person"=>{"name"=>{"fname"=>"James","middle"=>"T","lname"=>"Kirk"} which does not get handled by Person.create nor update_attributes like an association would.
Gives the following error
undefined method `fname' for {"fname"=>"James", "middle"=>"T", "lname"=>"Kirk"}:ActiveSupport::HashWithIndifferentAccess
How would you implement this?
Example
$ rails g scaffold person last_name:string first_name:string middle_name:string
lib/fullname.rb
class Fullname
attr_reader :fname, :middle, :lname
def initialize(fname, middle, lname)
#fname, #middle, #lname = fname, middle, lname
end
end
app/models/person.rb
class Person < ActiveRecord::Base
composed_of :name,
:class_name => 'Fullname',
:mapping =>
[ # database # Fullname
[:first_name, :fname],
[:middle_name, :middle],
[:last_name, :lname]
],
:allow_nil => true
end
app/views/people/_form.html.haml
= simple_form_for #person do |f|
= f.input :name, :as => :fullname
= f.submit 'Save'
app/inputs/fullname_input.rb
class FullnameInput < SimpleForm::Inputs::Base
def input
#builder.simple_fields_for attribute_name, :validate => false do |form|
[ 'First:', form.input_field(:fname, :size => 10),
'Middle:', form.input_field(:middle, :size => 5),
'Last:', form.input_field(:lname, :size => 10)
].join(' ').html_safe
end
end
end