🐛(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/migrations/0020_remove_document_is_public_and_more.py

@@ -0,0 +1,17 @@
+# Generated by Django 5.1.7 on 2025-03-14 14:03
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("core", "0019_alter_user_language_default_to_null"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="document",
+            name="has_deleted_children",
+            field=models.BooleanField(default=False),
+        ),
+    ]

src/backend/core/models.py

@@ -486,6 +486,7 @@ 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)
 
     _content = None
 
@@ -546,6 +547,12 @@ 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."""
@@ -903,7 +910,8 @@ 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

src/backend/core/tests/test_models_documents.py

@@ -1297,3 +1297,47 @@ def test_models_documents_restore_complex_bis(django_assert_num_queries):
 def test_models_documents_get_select_options(ancestors_links, select_options):
     """Validate that the "get_select_options" method operates as expected."""
     assert models.LinkReachChoices.get_select_options(ancestors_links) == select_options
+
+
+def test_models_documents_children_create_after_sibling_deletion():
+    """
+    It should be possible to create a new child after all children have been deleted.
+    """
+
+    root = factories.DocumentFactory()
+    assert root.numchild == 0
+    assert root.has_deleted_children is False
+    assert root.is_leaf() is True
+    child1 = factories.DocumentFactory(parent=root)
+    child2 = factories.DocumentFactory(parent=root)
+
+    root.refresh_from_db()
+    assert root.numchild == 2
+    assert root.has_deleted_children is False
+    assert root.is_leaf() is False
+
+    child1.soft_delete()
+    child2.soft_delete()
+    root.refresh_from_db()
+    assert root.numchild == 0
+    assert root.has_deleted_children is True
+    assert root.is_leaf() is False
+
+    factories.DocumentFactory(parent=root)
+    root.refresh_from_db()
+    assert root.numchild == 1
+    assert root.has_deleted_children is True
+    assert root.is_leaf() is False
+
+
+def test_models_documents_has_deleted_children():
+    """
+    A document should have its has_deleted_children attribute set to True if one of its children
+    has been solf deleted no matter if numchild is 0 or not.
+    """
+    root = factories.DocumentFactory()
+    child = factories.DocumentFactory(parent=root)
+    assert root.has_deleted_children is False
+    child.soft_delete()
+    root.refresh_from_db()
+    assert root.has_deleted_children is True