app/models/seminar_signup.rb

@@ -51,10 +51,9 @@ class SeminarSignup
       [l, (tmp ? tmp : unit)]
     end.to_h
     if self.serial_number.nil?
-      s = SeminarMain.find(self.seminar_main)
-      s.last_serial_number += 1
-      self.serial_number = s.last_serial_number
-    	s.save(:validate=>false)
+      last_serial_number = SeminarMain.where(:id=>self.seminar_main_id).max(:last_serial_number).to_i
+      self.serial_number = last_serial_number + 1
+      SeminarMain.where(:id=>self.seminar_main_id).update_all({"$inc"=>{"last_serial_number"=>1}})
     end
   end
   after_destroy do