型番を間違えたようです
class LineItemAddons < ActiveRecord::Base
belongs_to :line_item
has_one :addon_type_value
attr_accessible :quantity, :received, :snapshot_price, :status, :line_item_id, :addon_type_value_id
class LineItem < ActiveRecord::Base
belongs_to :design
belongs_to :cart
belongs_to :designer_order
belongs_to :return
belongs_to :return_designer_order
has_many :line_item_addons
class AddonTypeValue < ActiveRecord::Base
belongs_to :design
belongs_to :addon_type
belongs_to :line_item_addon
attr_accessible :description, :name, :position, :price, :addon_type_id, :design_id, :quantity
line_item_addons の初期化されていない定数を取得する