Express: Referencing the parent within the route definition for a child - node.js

Question

I am currently studying https://thinkster.io/mean-stack-tutorial/, but am puzzled about the route definition for a child element. For example, in the following...
// create a new comment
router.post('/posts/:post/comments', function(req, res, next) {
var comment = new Comment(req.body);
comment.post = req.post;
comment.save(function(err, comment){
if(err){ return next(err); }
req.post.comments.push(comment);
req.post.save(function(err, post) {
if(err){ return next(err); }
res.json(comment);
});
});
});
... we are trying to define the route for adding a new comment for a particular post. As you can see, we are required to link the parent to the comment...
comment.post = req.post;
... and we are also required to, understandably, save the parent within the comment save function. However, when comes to updating the comment (i.e. increasing the upvotes for a comment) we are only required to do...
// upvote a comment
router.put('/posts/:post/comments/:comment/upvote', function(req, res, next) {
req.comment.upvote(function(err, comment){
if (err) { return next(err); }
res.json(comment);
});
});
... where 'upvote' is a custom schema method...
CommentSchema.methods.upvote = function(cb) {
this.upvotes += 1;
this.save(cb);
};
... why is that we are not required to reference the parent object 'post' here at all? I mean how does the system know exactly which comment to load? I understand that ':post' refers to the parent post, however, we are not even referencing that pre-loaded parent within the method... so how is it being used? Are the correct set of comments automatically loaded from the pre-loaded post parent object, without our need to do this explicitly?
Any guidance would be appreciated.

Answer 1

according with the tutorial :
router.param('post', function(req, res, next, id) {
var query = Post.findById(id);
query.exec(function (err, post){
if (err) { return next(err); }
if (!post) { return next(new Error('can\'t find post')); }
req.post = post;
return next();
});
});
This code will take params from route: post/:post , so basically if you test the route : post/1 , it will set into request object(req) the post with id:1