解决选课函数中存在的竞争问题

加了相应的unique constraint，并且使用原子的update_or_create，避免在数据库中产生两个相同的记录

app/course_utils.py

@@ -608,16 +608,13 @@ def registration_status_change(course_id: int, user: NaturalPerson,
             and course_status != Course.Status.STAGE2):
         return wrong("在非选课阶段不能选课！")
 
-    need_to_create = False
-
     if action == "select":
         if CourseParticipant.objects.filter(course_id=course_id,
                                             person=user).exists():
             participant_info = CourseParticipant.objects.get(
                 course_id=course_id, person=user)
             cur_status = participant_info.status
         else:
-            need_to_create = True
             cur_status = CourseParticipant.Status.UNSELECT
 
         if course_status == Course.Status.STAGE1:
@@ -681,15 +678,11 @@ def registration_status_change(course_id: int, user: NaturalPerson,
                     course.current_participants += 1
                     course.save()
 
-                    # 由于不同用户之间的状态不共享，这个更新应该可以不加锁
-                    if need_to_create:
-                        CourseParticipant.objects.create(course_id=course_id,
-                                                         person=user,
-                                                         status=to_status)
-                    else:
-                        CourseParticipant.objects.filter(
-                            course_id=course_id,
-                            person=user).update(status=to_status)
+                    CourseParticipant.objects.update_or_create(
+                        course_id = course_id,
+                        person = user,
+                        defaults = {"status": to_status}
+                    )
                     succeed("选课成功！", context)
     except:
         return context

app/migrations/0009_courseparticipant_unique_course_selection_record.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.9 on 2025-01-17 20:52
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("app", "0008_pool_activity_alter_poolitem_exchange_attributes_and_more"),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name="courseparticipant",
+            constraint=models.UniqueConstraint(
+                fields=("course", "person"), name="Unique course selection record"
+            ),
+        ),
+    ]

app/models.py

@@ -1580,6 +1580,9 @@ class CourseParticipant(models.Model):
     class Meta:
         verbose_name = "4.课程报名情况"
         verbose_name_plural = verbose_name
+        constraints = [
+            models.UniqueConstraint(fields = ['course', 'person'], name='Unique course selection record')
+        ]
 
     course = models.ForeignKey(Course, on_delete=models.CASCADE,
                                related_name="participant_set")