Mongoose: Populate a tree with parent referencing top-down - node.js

Question

As I have stated in another question, I am working on a project involving a tree.
The tree uses parent referencing, so every node has the id of its parent
I need to load the tree top-down (from root to children) from the db and replace the parent references by children arrays (because the client needs them)
I've chosen this method, because I estimate 98% of the operation to be create/update on nodes (and this way I only have to create 1 node on update, instead of also updating the parent to add the child to the array) and only about 2% to be read operations (I only have to read the complete tree, there is no use case for reading parts or subtrees)
The Tree models is:
const mongoose = require("mongoose");
const Promise = require("bluebird");
mongoose.Promise = Promise;
const Node = require("./node-model");
const TreeSchema = new mongoose.Schema({
root: { type: Schema.Types.ObjectId, ref: 'Node' },
});
And the Node model:
const mongoose = require("mongoose");
const Promise = require("bluebird");
mongoose.Promise = Promise;
const NodeSchema = new mongoose.Schema({
parent: Schema.Types.ObjectId,
children: [], // to be populated on loading the tree
data: {
d1: String,
//...
}
});
NodeSchema.methods.populateTree = function() {
return this.constructor.find({ parent: this._id }).exec()
.then(function(arrayOfChildren) {
return Promise.each(arrayOfChildren, function(child){
this.children.push(child); // PROBLEM: 'this' is undfined here!
delete child.parent; // delete parent reference because JSON has problems with circular references
return child.populateTree();
});
});
}
Also, there is a tree container:
const TreeContainerSchema = new mongoose.Schema({
owner: { type: Schema.Types.ObjectId, ref: 'User', required: true },
tree: { type: Schema.Types.ObjectId, ref: 'Tree' },
});
I'm trying to load the complete tree (in his container) to send it back to the client as JSON as follows:
getTreeContainerById = function(req, res) {
var promise = TreeContainer.
findById(req.params.id).
populate("owner", "name"). // only include name
populate({
path: "tree",
populate: {
path: "root",
populate: "data"
}
}).exec();
promise.then(function(treeContainer){
return treeContainer.tree.root.populateTree()
.then(function(){ return treeContainer });
}).then(function(treeContainer) {
// I need the tree container here to send it back to the client
res.json(treeContainer);
});
};
But this implementation isn't working. The problems I face are:
In the populateTree schema method, I can't access the current node through "this" (it is undefined) but I need the reference somehow to add the children to the array
If I try child.parent.children.push instead, this also isn't working, because I only have the id of the parent (in child.parent) and not the entity (and I don't think it is the correct approach to load it again from the database)
In an earlier version, I had the problem, that the JSON was send back to the client, before the tree was completely populated, but I think i solved this through the use of the schema method
In general, I don't know, if this is the correct approach to solve my problem (populate the children references and delete the parent references in my tree) or if there is a more appropriate solution
I hope, I could make my problem clear. Any help is much appreciated!

Answer 1

With populateTree as follows it works:
NodeSchema.methods.populateTree = function() {
var node = this;
return this.constructor.find({ parent: this._id }).exec()
.then(function(arrayOfChildren) {
return Promise.each(arrayOfChildren, function(child){
node.children.push(child);
child.parent = null;
return child.populateTree();
});
});
}
Thanks to #danh who suggested the same!