Prevent double active infractions with constraint

https://github.com/python-discord/site/issues/273

This commits adds a UniqueConstraint for active infractions on a
combination of the `user` and `type` field. This means that a user
can only have one active infraction of a given type in the database
at any time.

I've also added tests to make sure that this behaves as expected.

3 files changed, 111 insertions, 2 deletions

diff --git a/pydis_site/apps/api/migrations/0045_add_infractions_unique_constraints_active.py b/pydis_site/apps/api/migrations/0045_add_infractions_unique_constraints_active.py
new file mode 100644
index 00000000..bacb56b8
--- /dev/null
+++ b/pydis_site/apps/api/migrations/0045_add_infractions_unique_constraints_active.py
@@ -0,0 +1,17 @@
+# Generated by Django 2.2.6 on 2019-10-07 18:27
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('api', '0044_active_infractions_migration'),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name='infraction',
+            constraint=models.UniqueConstraint(condition=models.Q(active=True), fields=('user', 'type'), name='unique_active_infraction_per_type_per_user'),
+        ),
+    ]
diff --git a/pydis_site/apps/api/models/bot/infraction.py b/pydis_site/apps/api/models/bot/infraction.py
index dfb32a97..108fd3a2 100644
--- a/pydis_site/apps/api/models/bot/infraction.py
+++ b/pydis_site/apps/api/models/bot/infraction.py
@@ -71,3 +71,10 @@ class Infraction(ModelReprMixin, models.Model):
         """Defines the meta options for the infraction model."""
 
         ordering = ['-inserted_at']
+        constraints = (
+            models.UniqueConstraint(
+                fields=["user", "type"],
+                condition=models.Q(active=True),
+                name="unique_active_infraction_per_type_per_user"
+            ),
+        )
diff --git a/pydis_site/apps/api/tests/test_infractions.py b/pydis_site/apps/api/tests/test_infractions.py
index 8279b6a6..69d9ebea 100644
--- a/pydis_site/apps/api/tests/test_infractions.py
+++ b/pydis_site/apps/api/tests/test_infractions.py
@@ -1,6 +1,7 @@
 from datetime import datetime as dt, timedelta, timezone
 from urllib.parse import quote
 
+from django.db.utils import IntegrityError
 from django_hosts.resolvers import reverse
 
 from .base import APISubdomainTestCase
@@ -167,6 +168,12 @@ class CreationTests(APISubdomainTestCase):
             discriminator=1,
             avatar_hash=None
         )
+        cls.second_user = User.objects.create(
+            id=6,
+            name='carl',
+            discriminator=2,
+            avatar_hash=None
+        )
 
     def test_accepts_valid_data(self):
         url = reverse('bot:infraction-list', host='api')
@@ -390,6 +397,82 @@ class CreationTests(APISubdomainTestCase):
         second_response = self.client.post(url, data=second_active_infraction)
         self.assertEqual(second_response.status_code, 201)
 
+    def test_unique_constraint_raises_integrity_error_on_second_active_of_same_type(self):
+        """Do we raise `IntegrityError` for the second active infraction of a type for a user?"""
+        Infraction.objects.create(
+            user=self.user,
+            actor=self.user,
+            type="ban",
+            active=True,
+            reason="The first active ban"
+        )
+        with self.assertRaises(IntegrityError):
+            Infraction.objects.create(
+                user=self.user,
+                actor=self.user,
+                type="ban",
+                active=True,
+                reason="The second active ban"
+            )
+
+    def test_unique_constraint_accepts_active_infraction_after_inactive_infraction(self):
+        """Do we accept an active infraction if the others of the same type are inactive?"""
+        Infraction.objects.create(
+            user=self.user,
+            actor=self.user,
+            type="ban",
+            active=False,
+            reason="The first inactive ban"
+        )
+        Infraction.objects.create(
+            user=self.user,
+            actor=self.user,
+            type="ban",
+            active=False,
+            reason="The second inactive ban"
+        )
+        Infraction.objects.create(
+            user=self.user,
+            actor=self.user,
+            type="ban",
+            active=True,
+            reason="The first active ban"
+        )
+
+    def test_unique_constraint_accepts_second_active_of_different_type(self):
+        """Do we accept a second active infraction of a different type for a given user?"""
+        Infraction.objects.create(
+            user=self.user,
+            actor=self.user,
+            type="ban",
+            active=True,
+            reason="The first active ban"
+        )
+        Infraction.objects.create(
+            user=self.user,
+            actor=self.user,
+            type="mute",
+            active=True,
+            reason="The first active mute"
+        )
+
+    def test_unique_constraint_accepts_active_infractions_for_different_users(self):
+        """Do we accept two active infractions of the same type for two different users?"""
+        Infraction.objects.create(
+            user=self.user,
+            actor=self.user,
+            type="ban",
+            active=True,
+            reason="An active ban for the first user"
+        )
+        Infraction.objects.create(
+            user=self.second_user,
+            actor=self.second_user,
+            type="ban",
+            active=False,
+            reason="An active ban for the second user"
+        )
+
 
 class ExpandedTests(APISubdomainTestCase):
     @classmethod
@@ -403,12 +486,14 @@ class ExpandedTests(APISubdomainTestCase):
         cls.kick = Infraction.objects.create(
             user_id=cls.user.id,
             actor_id=cls.user.id,
-            type='kick'
+            type='kick',
+            active=False
         )
         cls.warning = Infraction.objects.create(
             user_id=cls.user.id,
             actor_id=cls.user.id,
-            type='warning'
+            type='warning',
+            active=False,
         )
 
     def check_expanded_fields(self, infraction):