user topic: Add case insensitive UNIQUE constraint.

This will allow us to rely on the database to detect duplicate
`UserTopic`s (with the same `topic_name` with different cases)
and thus correctly throw IntegrityErrors when expected.
This is also important from a correctness point of view, since as
of now, when checking if topic is muted or requesting the backend for
muting a topic, the frontend does not check for case insensitivity.

There might exist duplicate UserTopics (in a case insensitive sense)
which need are removed before creating the new index.
The migration was tested manually using `./manage.py shell`.

zerver/migrations/0429_user_topic_case_insensitive_unique_toghether.py

@@ -0,0 +1,32 @@
+# Generated by Django 4.1.6 on 2023-02-11 05:16
+
+import django.db.models.functions.text
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0428_remove_realm_email_address_visibility"),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name="usertopic",
+            unique_together=set(),
+        ),
+        # Before adding the constraint, remove any pre-existing duplicates from the table
+        migrations.RunSQL(
+            """
+        DELETE FROM zerver_usertopic WHERE id NOT IN (SELECT max(id) FROM zerver_usertopic GROUP BY (user_profile_id, stream_id, upper(topic_name::text)));
+        """
+        ),
+        migrations.AddConstraint(
+            model_name="usertopic",
+            constraint=models.UniqueConstraint(
+                models.F("user_profile"),
+                models.F("stream"),
+                django.db.models.functions.text.Lower("topic_name"),
+                name="usertopic_case_insensitive_topic_uniq",
+            ),
+        ),
+    ]

zerver/models.py

@@ -43,7 +43,7 @@ from django.core.validators import MinLengthValidator, RegexValidator, URLValida
 from django.db import models, transaction
 from django.db.backends.base.base import BaseDatabaseWrapper
 from django.db.models import CASCADE, Exists, F, OuterRef, Q, Sum
-from django.db.models.functions import Upper
+from django.db.models.functions import Lower, Upper
 from django.db.models.query import QuerySet
 from django.db.models.signals import post_delete, post_save, pre_delete
 from django.db.models.sql.compiler import SQLCompiler
@@ -2648,7 +2648,14 @@ class UserTopic(models.Model):
     visibility_policy = models.SmallIntegerField(choices=visibility_policy_choices, default=MUTED)
 
     class Meta:
-        unique_together = ("user_profile", "stream", "topic_name")
+        constraints = [
+            models.UniqueConstraint(
+                "user_profile",
+                "stream",
+                Lower("topic_name"),
+                name="usertopic_case_insensitive_topic_uniq",
+            ),
+        ]
 
         indexes = [
             models.Index("stream", Upper("topic_name"), name="zerver_mutedtopic_stream_topic"),