refactor(reaction): 收敛为单选接口

reaction/migrations/0003_keep_one_reaction_per_user.py

@@ -0,0 +1,32 @@
+from django.db import migrations, models
+from django.db.models import Min, Subquery
+
+
+def keep_earliest(apps, schema_editor):
+    """一人一题只保留最早的一条表情，多余的删掉。
+
+    以前上限是 3 个，改成单选后这些老数据仍在给全站计数贡献数字。
+    同一次提交的 create_time 相同，所以按自增 id 认「最早」。
+    """
+    Reaction = apps.get_model("reaction", "Reaction")
+    keep_ids = Reaction.objects.values("user_id", "problem_id").annotate(keep=Min("id")).values("keep")
+    Reaction.objects.exclude(id__in=Subquery(keep_ids)).delete()
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("reaction", "0002_delete_comment"),
+    ]
+
+    operations = [
+        # 删掉的数据回不来，回滚只当无事发生
+        migrations.RunPython(keep_earliest, migrations.RunPython.noop),
+        migrations.AlterUniqueTogether(
+            name="reaction",
+            unique_together=set(),
+        ),
+        migrations.AddConstraint(
+            model_name="reaction",
+            constraint=models.UniqueConstraint(fields=("problem", "user"), name="reaction_problem_user_unique"),
+        ),
+    ]