reaction: Fix missing unique constraint on Reactions model.

This fixes a missing unique constraint on the Reactions data model
state when using multiple aliases for an emoji code.  As with any
missing unique constraints, we first need to apply a migration that
eliminates violations of the rule; in this case, deleting the
duplicates is correct.

Added unique constraint for "user_profile", "message",
"reaction_type", "emoji_code".

Fixes #15347.

zerver/migrations/0287_clear_duplicate_reactions.py

@@ -0,0 +1,35 @@
+from django.db import migrations
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+from django.db.models import Count
+
+
+def clear_duplicate_reactions(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    """Zulip's data model for reactions has enforced via code,
+    nontransactionally, that they can only react with one emoji_code
+    for a given reaction_type.  This fixes any that were stored in the
+    database via a race; the next migration will add the appropriate
+    database-level unique constraint.
+    """
+    Reaction = apps.get_model('zerver', 'Reaction')
+
+    duplicate_reactions = Reaction.objects.all().values(
+        "user_profile_id", "message_id", "reaction_type", "emoji_code").annotate(
+            Count('id')).filter(id__count__gt=1)
+    for duplicate_reaction in duplicate_reactions:
+        duplicate_reaction.pop('id__count')
+        to_cleanup = Reaction.objects.filter(**duplicate_reaction)[1:]
+        for reaction in to_cleanup:
+            reaction.delete()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0286_merge_0260_0285'),
+    ]
+
+    operations = [
+        migrations.RunPython(clear_duplicate_reactions,
+                             reverse_code=migrations.RunPython.noop,
+                             elidable=True),
+    ]

zerver/migrations/0288_reaction_unique_on_emoji_code.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2.13 on 2020-06-19 08:16
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0287_clear_duplicate_reactions'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='archivedreaction',
+            unique_together={('user_profile', 'message', 'emoji_name'), ('user_profile', 'message', 'reaction_type', 'emoji_code')},
+        ),
+        migrations.AlterUniqueTogether(
+            name='reaction',
+            unique_together={('user_profile', 'message', 'emoji_name'), ('user_profile', 'message', 'reaction_type', 'emoji_code')},
+        ),
+    ]

zerver/models.py

@@ -1893,7 +1893,8 @@ class AbstractReaction(models.Model):
 
     class Meta:
         abstract = True
-        unique_together = ("user_profile", "message", "emoji_name")
+        unique_together = (("user_profile", "message", "emoji_name"),
+                           ("user_profile", "message", "reaction_type", "emoji_code"))
 
 class Reaction(AbstractReaction):
     message: Message = models.ForeignKey(Message, on_delete=CASCADE)