Mongodb, getting the id of the newly pushed embedded object

Question

I have embedded comments in a posts model. I am using mongoosejs. After pushing a new comment in a post, I want to access the id of the newly added embedded comment. Not sure how to get it.

Here is how the code looks like.

var post = Post.findById(postId,function(err,post){

   if(err){console.log(err);self.res.send(500,err)}

   post.comments.push(comment);

   post.save(function(err,story){
       if(err){console.log(err);self.res.send(500,err)}
           self.res.send(comment);
   })


});

In the above code, the id of the comment is not returned. Note there is a _id field which is created in the db.

The schema looks like

var CommentSchema = new Schema({
  ...
})

var PostSchema = new Schema({
    ...
    comments:[CommentSchema],
    ...
});

Answer 1

A document's _id value is actually assigned by the client, not the server. So the _id of the new comment is available right after you call:

post.comments.push(comment);

The embedded doc pushed to post.comments will have its _id assigned as it's added, so you can pull it from there:

console.log('_id assigned is: %s', post.comments[post.comments.length-1]._id);

Answer 2

You can manually generate the _id then you don't have to worry about pulling it back out later.

var mongoose = require('mongoose');
var myId = mongoose.Types.ObjectId();

// then set the _id key manually in your object

_id: myId

// or

myObject[_id] = myId

// then you can use it wherever

Answer 3

_id field is generated at client side, you can get the id of the embedded document by comment.id

sample

 > var CommentSchema = new Schema({
     text:{type:String}
  })

 > var CommentSchema = new mongoose.Schema({
     text:{type:String}
 })

 > var Story = db.model('story',StorySchema)
 > var Comment = db.model('comment',CommentSchema)
 > s= new Story({title:1111})
   { title: '1111', _id: 5093c6523f0446990e000003, comments: [] }
 > c= new Comment({text:'hi'})
   { text: 'hi', _id: 5093c65e3f0446990e000004 }
 > s.comments.push(c)
 > s.save()

verify in mongo db shell

    > db.stories.findOne()
{
    "title" : "1111",
    "_id" : ObjectId("5093c6523f0446990e000003"),
    "comments" : [
        {
            "_id" : ObjectId("5093c65e3f0446990e000004"),
            "text" : "hi"
        }
    ],
    "__v" : 0
}