Nested_form with scoped selects of the same has_many through relation

I am working on a product admin section. I have productvariants, which can have many productattributevalues through productvariantdetails.

productattributevalues (green, yellow, S, XL) belongs_to productattributes where I have specified attributes such as: color, size and material.

I would like to have a nested form on my productvariants/_form where I can choose (optionally) if a productvariant has an productattributevalue of: size or color applied.

I changed my models and introduced two aliases for productattributevalues. One for each attribute.

class Productvariant < ActiveRecord::Base
  has_many :productattributevalues
  has_many :productvariantdetails
  has_many :productattributevalues, :through => :productvariantdetails
  has_many :productattributecolors, :through => :productvariantdetails, :source => :productattributevalue
  has_many :productattributesizes, :through => :productvariantdetails, :source => :productattributevalue
  accepts_nested_attributes_for :productvariantdetails, :allow_destroy => true
end

class Productattributevalue < ActiveRecord::Base
  attr_accessible :value, :productattribute_id
  scope :product_color, joins(:productattribute).where('productattributes.attributename' => 'color')
  scope :product_size, joins(:productattribute).where('productattributes.attributename' => 'size')
  belongs_to :productattribute
end

This is my view

<%= form_for [:admin, @productvariant], :html => {'role' => 'form' } do |f| %>
    .
    .
    .
  <%= f.fields_for : productattributecolors do |ff| %>
      <%= ff.label "Productattribute: Color" %>
      <%= ff.select(:productattributecolor, Productattributevalue.product_color.uniq.order('id asc').map{|s| [s.value, s.id]}, {:selected => params[:productattributecolor], :include_blank => false, :prompt => "Select color"}, {}) %>
  <% end %>
  <%= f.fields_for : productattributesizes do |ff| %>
      <%= ff.label "Productattribute: Size" %>
      <%= ff.select(:productattributesize, Productattributevalue.product_size.uniq.order('id asc').map{|s| [s.value, s.id]}, {:selected => params[:productattributesize], :include_blank => false, :prompt => "Select size"}, {}) %>
  <% end %>
    .
    .
    .
    <%= f.submit "Save" %>
<% end %>

How can I get this working?

Thank you in advance!