✨(backend) add django-treebeard to allow tree structure on documents

We choose to use Django-treebeard for its quality, performance and
stability. Adding tree structure to documents is as simple as
inheriting from the MP_Node class.

src/backend/core/factories.py

@@ -46,6 +46,23 @@ class UserFactory(factory.django.DjangoModelFactory):
             UserTemplateAccessFactory(user=self, role="owner")
 
 
+class ParentNodeFactory(factory.declarations.ParameteredAttribute):
+    """Custom factory attribute for setting the parent node."""
+
+    def generate(self, step, params):
+        """
+        Generate a parent node for the factory.
+
+        This method is invoked during the factory's build process to determine the parent
+        node of the current object being created. If `params` is provided, it uses the factory's
+        metadata to recursively create or fetch the parent node. Otherwise, it returns `None`.
+        """
+        if not params:
+            return None
+        subfactory = step.builder.factory_meta.factory
+        return step.recurse(subfactory, params)
+
+
 class DocumentFactory(factory.django.DjangoModelFactory):
     """A factory to create documents"""
 
@@ -54,6 +71,8 @@ class DocumentFactory(factory.django.DjangoModelFactory):
         django_get_or_create = ("title",)
         skip_postgeneration_save = True
 
+    parent = ParentNodeFactory()
+
     title = factory.Sequence(lambda n: f"document{n}")
     content = factory.Sequence(lambda n: f"content{n}")
     creator = factory.SubFactory(UserFactory)
@@ -64,6 +83,21 @@ class DocumentFactory(factory.django.DjangoModelFactory):
         [r[0] for r in models.LinkRoleChoices.choices]
     )
 
+    @classmethod
+    def _create(cls, model_class, *args, **kwargs):
+        """
+        Custom creation logic for the factory: creates a document as a child node if
+        a parent is provided; otherwise, creates it as a root node.
+        """
+        parent = kwargs.pop("parent", None)
+
+        if parent:
+            # Add as a child node
+            return parent.add_child(instance=model_class(**kwargs))
+
+        # Add as a root node
+        return model_class.add_root(instance=model_class(**kwargs))
+
     @factory.post_generation
     def users(self, create, extracted, **kwargs):
         """Add users to document from a given list of users with or without roles."""