Nested documents in mongodb implementation, just like Reddit comments - node.js

Question

In my project, I would like to implement a comment section which consists of list of comments.
const myschema= mongoose.Schema({
_id: mongoose.Schema.Types.ObjectId,
//other fields
comments : [comment]
},{collection : 'TABLE_NAME'} );
const comment= mongoose.Schema({
_id : mongoose.Schema.Types.ObjectId,
commentid: String, // id consists of uuid1()
senderid: Number, //user's id
body: String, // useful information
parent: String,// parent id consists of uuid1()
children: [] // I couldn't add children = [comment], it gives me an error,
//instead, I make it empty and will fill it when a comment comes
})
In the request tab, I will receive a JSON in the following format:
{
"userid": "NUMBERID HERE",
"comment": "COMMENT BODY",
"parent" : "Comment's parent id"
}
I would like to add a comment which can be a child of another comment. How can I search and find the appropriate position?
If there are no parent in JSON body, I'm doing this:
// import comment somewhere at the beginning
.then(doc =>{
var newc= new comment();
newc.cid = uuidv1();
newc.sender = req.body.userid;
newc.body = req.body.comment;
newc.parent = "";
newc.children = "";
doc.comments.push(newc);
// save to DB
doc
.save()
.then(docres =>{
res.status(200).json(docres);
})
.catch(err => {
res.status(500).json({error: err});
})
}
I have no idea how to find a comment that resides in a deep level

Answer 1

You cannot search for an array element or object property given an unspecified, arbitrarily-nested depth. It just isn't possible. You would need to instead handle this in the application layer. Denormalizing your data is fine in many cases, but arbitrary nesting depths isn't a recommended use case for data denormalization, especially since you can't index efficiently!
If you want a pure MongoDB solution, then you'll need a different document structure. I would recommend taking a look at the documentation, particularly the section concerning an array of ancestors, in order to properly model your data.

Answer 2

I have found a solution.
The manually traversing the comments and inserting in the right place works. However, it only works up to some level. In my case, I can insert a comment below comment and save it. I can also insert a 3rd deep level comment and see the JSON dump of the object that I have inserted or even wrap it in an HTTP response object and send to the client. But the database does not save this update.
What I did is, after inserting the comment in the correct place I added this code before saving to the database.
doc.markModified('comments'); // this is added
doc.save().then(/*prepare response*/);
Somehow the MongoDB or mongoose or javascript interpreter knows the document has been changed and save the updated version. If the markmodified part is not specified, probably the compiler thinks that the object has not been modified and skips it.
Hopefully, this helps people who encounter this issue.
This is my implementation
// find the document which comment is going to be inserted
MyDOC.findOne({'id' : req.params.id})
.exec()
.then(doc =>{
// if the comment has a parent, it should be inserted in nested
if(req.body.parent != null && req.body.parent != undefined)
{
// find correct position and insert
findComment(doc.comments, req.body);
doc.markModified('comments');
doc
.save()
.then(docres =>{
res.status(200).json(docres);
})
.catch(err => {
console.log(err);
res.status(500).json({error: err});
})
}
else
{
//Add comment to the root level
var comment= new Comment();
comment.cid = uuidv1();
comment.body = req.body.comment;
comment.parent = "";
doc.comments.push(comment);
doc.markModified('comments');
// save to DB
doc
.save()
.then(docres =>{
res.status(200).json(docres);
})
.catch(err => {
console.log(err);
res.status(500).json({error: err});
})
}
})
function findComment(comment, body)
{
comment.forEach(element => {
if(element.children.length != 0)
{
if(element.cid === body.parent)
{
// found, insert
var comment= new Comment();
comment.cid = uuidv1();
comment.body = body.comment;
comment.parent = body.parent;
element.children.push(comment);
return true;
}
if(findComment(element.children, body, usr))
return true;
}
else
{
// this comment does not have children. If this comments id and the body's parent is equal, add it
if(element.cid === body.parent)
{
// found, insert
var comment= new Comment();
comment.cid = uuidv1();
comment.body = body.comment;
comment.parent = body.parent;
element.children.push(comment);
return true;
}
return false;
}
});
}