Allow nested collections

core/models.py

@@ -222,7 +222,8 @@ class Collection(models.Model):
 
 class CollectionSection(models.Model):
     collection = models.ForeignKey(Collection, on_delete=models.CASCADE, related_name='sections')
-    item = models.ForeignKey(Item, on_delete=models.PROTECT, related_name='collection_sections')
+    item = models.ForeignKey(Item, on_delete=models.PROTECT, blank=True, null=True, related_name='collection_sections')
+    sub_collection = models.ForeignKey(Collection, on_delete=models.PROTECT, blank=True, null=True, related_name='containing_sections')
     title = models.CharField(max_length=200, blank=True)
     position = models.PositiveIntegerField(default=0)
     created_at = models.DateTimeField(auto_now_add=True)
@@ -230,12 +231,17 @@ class CollectionSection(models.Model):
     class Meta:
         ordering = ['position', 'id']
         constraints = [
-            models.UniqueConstraint(fields=['collection', 'item'], name='unique_collection_item'),
+            models.CheckConstraint(
+                check=(models.Q(item__isnull=False, sub_collection__isnull=True) | models.Q(item__isnull=True, sub_collection__isnull=False)),
+                name='collection_section_exactly_one_target',
+            ),
+            models.UniqueConstraint(fields=['collection', 'item'], condition=models.Q(item__isnull=False), name='unique_collection_item'),
+            models.UniqueConstraint(fields=['collection', 'sub_collection'], condition=models.Q(sub_collection__isnull=False), name='unique_collection_sub_collection'),
             models.UniqueConstraint(fields=['collection', 'position'], name='unique_collection_position'),
         ]
 
     def __str__(self):
-        return f'{self.collection}: {self.title or self.item}'
+        return f'{self.collection}: {self.title or self.item or self.sub_collection}'
 
 
 class UserPreference(models.Model):