Context
We are working on a project using treebeard's NS_Node implementation. We are building a system to recursively copy a folder structure into our database for tracking.

Issue
We found that recursively calling add_child resulted in an invalid tree data structure, with problems such as a child with multiple parents. After some digging, we discovered that this error was the result of a caching problem in add_child. We were able to implement a workaround for this bug by calling refresh_from_db() on the parent node before calling node.add_child().

We additionally attempted using the load_bulk function to see if this was an undocumented difference between add_child and load_bulk and found that this is a breaking bug for this function. load_bulk errors with
AttributeError: 'NoneType' object has no attribute '_cached_parent_obj'

Reproducible Example of load_bulk
To recreate the load_bulk issue on a model Folder(NS_Node):

new_root = Folder.add_root(name="new_root")
a_id = uuid.uuid4()
b_id = uuid.uuid4()
Folder.load_bulk([{"data": {"name": "a"}, "id": a_id, "children": [{"data": {"name": "b"}, "id": b_id}]}], new_root, True)

Reproducible Example of add_child
Given file structure:

A
 - B
 - C
   - File 1

We would expect to have three folders; A, children B and C. File 1 would be within Folder C. Instead, we see that C.get_parent() is B. Our implementation (with the workaround) is below:

def create_folders_and_files(tree, parent):
	parent.refresh_from_db() # THIS IS THE WORKAROUND
	new_folder = parent.add_child(name=tree['name'])
	for file, full_name in tree['files']:
		# create file within the folder you just made
	for folder in tree['folders']:
		create_folders_and_files(folder, new_folder)

create_folders_and_files(tree, root_node)