🐛(back) keep info if document has deleted children

With the soft delete feature, relying on the is_leaf method from the
treebeard is not accurate anymore. To determine if a node is a leaf, it
checks if the number of numchild is equal to 0. But a node can have soft
deleted children, then numchild is equal to 0, but it is not a leaf
because if we want to add a child we have to look for the last child to
compute a correct path. Otherwise we will have an error saying that the
path already exists.

src/backend/core/models.py

@@ -384,6 +384,7 @@ class Document(MP_Node, BaseModel):
     )
     deleted_at = models.DateTimeField(null=True, blank=True)
     ancestors_deleted_at = models.DateTimeField(null=True, blank=True)
+    has_deleted_children = models.BooleanField(default=False)
     duplicated_from = models.ForeignKey(
         "self",
         on_delete=models.SET_NULL,
@@ -465,6 +466,12 @@ class Document(MP_Node, BaseModel):
                 content_file = ContentFile(bytes_content)
                 default_storage.save(file_key, content_file)
 
+    def is_leaf(self):
+        """
+        :returns: True if the node is has no children
+        """
+        return not self.has_deleted_children and self.numchild == 0
+
     @property
     def key_base(self):
         """Key base of the location where the document is stored in object storage."""
@@ -886,7 +893,8 @@ class Document(MP_Node, BaseModel):
 
         if self.depth > 1:
             self._meta.model.objects.filter(pk=self.get_parent().pk).update(
-                numchild=models.F("numchild") - 1
+                numchild=models.F("numchild") - 1,
+                has_deleted_children=True,
             )
 
         # Mark all descendants as soft deleted