删除

app/models/course.rb

@@ -146,7 +146,7 @@ class Course < ApplicationRecord
   def teacher_group(user_id)
     data =
       if teacher_course_groups.exists?(user_id: user_id)
-        teacher_course_groups.joins(:course_group).where(user_id: user_id).distinct
+        teacher_course_groups.joins(:course_group).where(user_id: user_id)
           .pluck('course_groups.id', 'course_groups.name')
       else
         course_groups.pluck(:id, :name)

db/migrate/20200102030946_modify_course_group_for_courses.rb

@@ -11,7 +11,7 @@ class ModifyCourseGroupForCourses < ActiveRecord::Migration[5.2]
           cg.attachment_group_settings.update_all(course_group_id: g.id)
           cg.homework_group_reviews.update_all(course_group_id: g.id)
           cg.homework_group_settings.update_all(course_group_id: g.id)
-          cg.teacher_course_groups.update_all(course_group_id: g.id)
+          #cg.teacher_course_groups.update_all(course_group_id: g.id)
           cg.destroy!
         end
       end