refactor(dash)!: enforce uniqueness pset

Add the constraint unique_together = ("student", "unit") at a database
level

dashboard/migrations/0087_alter_pset_unique_together.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.5 on 2023-09-12 01:43
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("roster", "0103_alter_assistant_user_alter_invoice_created_at_and_more"),
+        ("core", "0057_userprofile_show_portal_instructions"),
+        ("dashboard", "0086_alter_uploadedfile_created_at"),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name="pset",
+            unique_together={("student", "unit")},
+        ),
+    ]

dashboard/models.py

@@ -194,6 +194,7 @@ class PSet(models.Model):
     class Meta:
         verbose_name = "PSet submission"
         verbose_name_plural = "PSet submissions"
+        unique_together = ("student", "unit")
 
     def __str__(self):
         return f"{self.student.name} submits {self.unit}"