迁移课程总时间长的字段

db/migrate/20190815064541_modify_class_period_for_courses.rb

@@ -1,5 +1,8 @@
 class ModifyClassPeriodForCourses < ActiveRecord::Migration[5.2]
   def change
+    Course.find_each do |c|
+      c.update_column(:class_period, c.class_period.to_i)
+    end
     change_column :courses, :class_period, :integer, :default => 0
   end
 end