Collection: also save the collection UID on the model itself.

This enables us to have db-constraints for making sure that UIDs are
unique, as well as being more efficient for lookups (which are very
common).

The UID should always be the same as the main_item.uid, though that's
easily enforced as neither of them is allowed to change.

django_etebase/models.py

@@ -43,6 +43,8 @@ class CollectionType(models.Model):
 
 class Collection(models.Model):
     main_item = models.OneToOneField("CollectionItem", related_name="parent", null=True, on_delete=models.SET_NULL)
+    # The same as main_item.uid, we just also save it here so we have DB constraints for uniqueness (and efficiency)
+    uid = models.CharField(db_index=True, unique=True, blank=False, max_length=43, validators=[UidValidator])
     owner = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
 
     stoken_annotation = stoken_annotation_builder(["items__revisions__stoken", "members__stoken"])
@@ -50,10 +52,6 @@ class Collection(models.Model):
     def __str__(self):
         return self.uid
 
-    @cached_property
-    def uid(self):
-        return self.main_item.uid
-
     @property
     def content(self):
         return self.main_item.content
@@ -76,20 +74,6 @@ class Collection(models.Model):
 
         return Stoken.objects.get(id=stoken_id).uid
 
-    def validate_unique(self, exclude=None):
-        super().validate_unique(exclude=exclude)
-        if exclude is None or "main_item" in exclude:
-            return
-
-        if (
-            self.__class__.objects.filter(owner=self.owner, main_item__uid=self.main_item.uid)
-            .exclude(id=self.id)
-            .exists()
-        ):
-            raise EtebaseValidationError(
-                "unique_uid", "Collection with this uid already exists", status_code=status.HTTP_409_CONFLICT
-            )
-
 
 class CollectionItem(models.Model):
     uid = models.CharField(db_index=True, blank=False, max_length=43, validators=[UidValidator])