Post same objectID in to different table - node.js

Question

I'm trying to post a data in my user then at the same time, post the _id of my user as a reference id in my donation table.
After I posted my data in the users table like this:
var User = require('../models/user');
var Blooddonation = require('../models/blooddonation');
router.post('/createBlooduser',function(req, res) {
var user = new User();
user.user_lastname = req.body.user_lastname;
user.status= "initial";
user.save(function(err) {});
});
How can I get the _id of the posted data and make a reference id in my donation table? Something like this:
**users.json**
{
_id:ObjectId("5c7e591eee959272cc5768cb"),
user_lastname:"Jhonson",
status:"initial"
}
**blooddonations.json**
{
donor_id:ObjectId("5c7e591eee959272cc5768cb")
}

Answer 1

The _id property is actually created as soon as you create new instance with a statement like new User(). So you can actually access that value before it's even stored in the collection, or at any time after instance creation really:
router.post('/createBlooduser',function(req, res) {
var user = new User();
user.user_lastname = req.body.user_lastname;
user.status= "initial";
user.save(function(err) {
if (err) throw err; // or really handle better
// You can also just create() rather than new Blooddonation({ donor_id: user._id })
Blooddonation.create({ donor_id: user._id }, function(err, donor) {
// check for errors and/or respond
})
});
});
Of if you might just want access to other properties that might "default on save", then you can access in the callback from save() or create():
router.post('/createBlooduser',function(req, res) {
User.create({
user_lastname: req.body.user_lastname;
status: "initial"
}, function(err, user) { // this time we pass from the callback
if (err) throw err; // or really handle better
Blooddonation.create({ donor_id: user._id }, function(err, donor) {
// check for errors and/or respond
});
});
});