Downloads: Enforce constraint that only one ReleaseFile per OS per Re…

…lease has "Download button" enabled. (#2137)

* add fixtures for downloads/releases

* enforce constraint that only one release file per os has download_button value

This is by far the most common breakage during CPython releases... stop it from being possible

* Raise a ValidationError if attempting to breach constraint on release/os/download_button

downloads/migrations/0008_auto_20220907_2102.py

@@ -0,0 +1,17 @@
+# Generated by Django 2.2.24 on 2022-09-07 21:02
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('downloads', '0007_auto_20220809_1655'),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name='releasefile',
+            constraint=models.UniqueConstraint(condition=models.Q(download_button=True), fields=('os', 'release'), name='only_one_download_per_os_per_release'),
+        ),
+    ]

downloads/models.py

@@ -2,6 +2,7 @@
 
 from django.urls import reverse
 from django.conf import settings
+from django.core.exceptions import ValidationError
 from django.db import models
 from django.db.models.signals import post_save
 from django.dispatch import receiver
@@ -332,7 +333,20 @@ class ReleaseFile(ContentManageable, NameSlugModel):
     filesize = models.IntegerField(default=0)
     download_button = models.BooleanField(default=False, help_text="Use for the supernav download button for this OS")
 
+    def validate_unique(self, exclude=None):
+        if self.download_button:
+            qs = ReleaseFile.objects.filter(release=self.release, os=self.os, download_button=True).exclude(pk=self.id)
+            if qs.count() > 0:
+                raise ValidationError("Only one Release File per OS can have \"Download button\" enabled")
+        super(ReleaseFile, self).validate_unique(exclude=exclude)
+
     class Meta:
         verbose_name = 'Release File'
         verbose_name_plural = 'Release Files'
         ordering = ('-release__is_published', 'release__name', 'os__name', 'name')
+
+        constraints = [
+            models.UniqueConstraint(fields=['os', 'release'],
+            condition=models.Q(download_button=True),
+            name="only_one_download_per_os_per_release"),
+        ]