选题没保存班级

app/controllers/graduation_topics_controller.rb

@@ -193,8 +193,8 @@ class GraduationTopicsController < ApplicationController
   def student_select_topic
     user_unaccept_topics = @course.student_graduation_topics.where(user_id: current_user.id, status: [0, 1])
     if user_unaccept_topics.size == 0
-      member_id = @course.course_members.find_by_user_id(current_user.id)
-      StudentGraduationTopic.create(course_id: @course.id, user_id: current_user.id, course_member_id: member_id,
+      member = @course.course_members.find_by_user_id(current_user.id)
+      StudentGraduationTopic.create(course_id: @course.id, user_id: current_user.id, course_member_id: member.try(:id),
                                     graduation_topic_id: @graduation_topic.id)
       @graduation_topic.update_attribute(:status, 1)
       normal_status("选题成功")