Model tree structure in mongo - mongodb

Question

I'm looking to build a Tree structure for an apps where I'll have to store multiple Tree for each user.
Each tree will be composed of documents for each node :
{
_id:1,
user_id:12345,
category_id:6789,
data:[]
}
So when I need to, I can access these data with a query looking for user_id and category_id
I was looking on mongoDB Docs and found that :
https://docs.mongodb.com/manual/applications/data-models-tree-structures/
Which is pretty interesting, but I have a few questions
Considering that I'll only search for full tree, which solutions is better?
Are child references better, or other structure might do the work better?
And if I use child's references, what is the best way to get all the tree?
Can it be done with a single request or do I have to recursively search for each child ?
I know I could get all docs in one query and kind of build the tree from there, is that a good idea?
EDIT:
So I tried with that:
[{
_id:1,
user_id:12345,
category_id:6789,
data:{name:"root"},
parent:null,
childs:[2,3]
},
{
_id:2,
user_id:12345,
category_id:6789,
data:{name:"child1"},
parent:1,
childs:[]
},
{
_id:3,
user_id:12345,
category_id:6789,
data:{name:"child2"},
parent:1,
childs:[4]
},
{
_id:4,
user_id:12345,
category_id:6789,
data:{name:"child2_1"},
parent:3,
childs:[]
}]
With both parent and children, I can easily find leaves and root when building the tree back. (I chosed to build it in the client app, and query the full tree at once)
The fact is I don't really use parent for now, so it looks "Overkill" to get a reference to the parents, but the query is fast enough, it just take some extra space. Maybe a simple "root" boolean could be better ? I really need some kind of advice with that.
I'm still up to some improvements, I'd like to get this working really fast because each users will have 0 to n tree with 0 to n nodes, and I don't want to mess the data structure for that.

Answer 1

This article shows a similar kind of solution : How to query tree structure recursively with MongoDB?
Other wise parent in consideration, you can query out in this way to convert it to tree
function list_to_tree(list) {
const map1 = new Map();
var node,
roots = [],
i;
for (i = 0; i < list.length; i += 1) {
map1.set(list[i]._id.toString(), i); // initialize the map
list[i].childs = []; // initialize the children
}
for (i = 0; i < list.length; i += 1) {
node = list[i];
if (node.parent) {
list[map1.get(node.parent.toString())].childs.push(node);
} else {
roots.push(node);
}
}
let newList = [];
for (let z = 0; z < roots.length; z++) {
newList.push(list.filter((x) => x._id === roots[z]._id)[0]);
}
return newList;
}