尝试修复 submission 中的竞争问题

contest/migrations/0010_auto_20190326_0201.py

@@ -0,0 +1,23 @@
+# Generated by Django 2.1.7 on 2019-03-26 02:01
+
+from django.conf import settings
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('contest', '0009_auto_20180501_0436'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='acmcontestrank',
+            unique_together={('user', 'contest')},
+        ),
+        migrations.AlterUniqueTogether(
+            name='oicontestrank',
+            unique_together={('user', 'contest')},
+        ),
+    ]

contest/models.py

@@ -74,6 +74,7 @@ class ACMContestRank(AbstractContestRank):
 
     class Meta:
         db_table = "acm_contest_rank"
+        unique_together = (("user", "contest"),)
 
 
 class OIContestRank(AbstractContestRank):
@@ -84,6 +85,7 @@ class OIContestRank(AbstractContestRank):
 
     class Meta:
         db_table = "oi_contest_rank"
+        unique_together = (("user", "contest"),)
 
 
 class ContestAnnouncement(models.Model):