加入课堂的接口调整

app/controllers/courses_controller.rb

@@ -1146,6 +1146,7 @@ class CoursesController < ApplicationController
           if existing_student.present?
             # 如果在该课堂已经存在学生身份，且邀请码为分班邀请码，则将其直接加入分班
             existing_student.update!(course_group_id: course_group.id) if course_group.present?
+            message = "您已是课堂成员"
           else
             correspond_teacher_exist = current_user.none_admin_teacher_of_course? course
             new_student = CourseMember.new(user_id: current_user.id, course_id: course.id, role: 4)