Add instance method :series_with_spec to Product

lib/shotengai/product.rb

@@ -74,6 +74,10 @@ module Shotengai
       Shotengai::Series.alive.find_by_id(default_series_id) || series.alive.first
     end
 
+    def series_with_spec val
+      series_class.query_spec_value_with_product(val, self)
+    end
+
     def meta
       super || {}
     end