Add :shotengai_series to product

lib/shotengai/product.rb

@@ -33,6 +33,7 @@ module Shotengai
     harray_accessor :spec_template, :remark_template
     template_with_value_getters :spec, :remark, value_in_template: true
 
+    has_many :shotengai_series, class_name: 'Shotengai::Series', foreign_key: :shotengai_product_id
     belongs_to :manager, polymorphic: true, optional: true#, touch: true
 
     default_scope { order(created_at: :desc) }
@@ -78,7 +79,7 @@ module Shotengai
     }
 
     def default_series
-      Shotengai::Series.alive.find_by_id(default_series_id) || series.alive.first
+      Shotengai::Series.alive.find_by_id(default_series_id) || shotengai_series.alive.first
     end
 
     def meta