Spree::Variant.class_eval do
  has_and_belongs_to_many  :assemblies, :class_name => "Spree::Product",
        :join_table => "spree_assemblies_parts",
        :foreign_key => "part_id", :association_foreign_key => "assembly_id"

  def assemblies_for(products)
    assemblies.where(id: products)
  end

  def part?
    assemblies.exists?
  end
end