Add an actual constraint making sure we can only have one pinned news article

We only had a gui-level prevention of this before...

Author: mhagander

pgweb/news/migrations/0011_pinnednewsarticle_uniqueness.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.10 on 2026-06-24 15:20
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('news', '0010_newspostingembargo'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='pinnednewsarticle',
+            name='uniqueness',
+            field=models.GeneratedField(db_persist=True, expression=models.Value(1), output_field=models.IntegerField(), unique=True),
+        ),
+    ]

pgweb/news/models.py

@@ -132,6 +132,7 @@ def get_field_description(self, f):
 
 
 class PinnedNewsArticle(models.Model):
+    uniqueness = models.GeneratedField(expression=models.Value(1), output_field=models.IntegerField(), db_persist=True, unique=True)
     pinnedarticle = models.ForeignKey(NewsArticle, null=True, blank=True, on_delete=models.SET_NULL)
     pinnedtoproviders = models.JSONField(null=False, blank=True, default=dict)