Best Firestore data model for file tree data - database

Question

I'm currently building an app that needs to store data with a similar structure as a file tree. It looks something like this:
{
"type": "folder",
"name": "folder A",
"private": false,
"updatedAt": 1231243,
"items": [
{
"type": "folder",
"name": "subfolder A",
"private": false,
"updatedAt": 1231243,
"items": [
{
"type": "file",
"name": "file1"
}
]
},
{
"type": "file",
"name": "file2"
}
]
}
What I'm aware so far, there are 3 ways of implementing this.
Just dump all the data in 1 doc
Create a subcollection for each items array
Create a flat folder structure and save the parent folder id for lookup
I'm looking for a way to get all of this data with as minimum query as possible, the ideal use case would be only having the root folder id and getting all the subfolder & items. But I'm not sure if that's possible.
Also, I'm planning to subscribe to the data in the future, so the file tree will be updated in real-time.
Please give me a suggestion on what should the data model looks like
Update 1
To give more clarification about the query that I will need:
I will only need to get the topmost folder (folder A) in this example and the items under it
I won't need to get the nested items directly
example: getting Subfolder A / File 2 without accessing Folder A

Answer 1

If all you need is to get the entire structure by some unique ID, just put it all in one document, and get() it using the known ID. You have a 1MB limit per document.
It can get more complicated if you need to access items within lists (which is not really possible to do without reading the entire document), so this would actually be a bad idea for that case.
You would also want to reconsider this if you intend to update the elements of this document frequently, as there is a limit of 1 write per second (sustained). You would want to split it up otherwise.