班级删除成员后，隐藏对应作品记录历史数据迁移

db/migrate/20161125024643_migrate_student_works.rb

@@ -0,0 +1,18 @@
+class MigrateStudentWorks < ActiveRecord::Migration
+  def up
+    count = Course.all.count / 30 + 2
+    transaction do
+      for i in 1 ... count do i
+        Course.page(i).per(30).each do |course|
+          homework_ids = course.homework_commons.blank? ? "(-1)" : "(" + course.homework_commons.map{|hw| hw.id}.join(",") + ")"
+          student_ids = course.student.blank? ? "(-1)" : "(" + course.student.map{|st| st.student_id}.join(",") + ")"
+          student_works = StudentWork.where("homework_common_id in #{homework_ids} and user_id not in #{student_ids}")
+          student_works.update_all(:is_delete => 1)
+        end
+      end
+    end
+  end
+
+  def down
+  end
+end