Mongoose object id is null - node.js

Question

I'm creating an object Registration (a Mongoose Schema) and I need to set its Id in User with this line: registrationId: registration._id});
However, the Id is still null, even though it's the callback function? When I check the database the Registration has and Id of course, but not in the callback. How can I set the Id of the Registration in User?
Edit2: changed to minimal example. This prints out two times null.
exports.create = function(req, res) {
Registration.create(req.body, function(err, registration) {
if(err) { return handleError(res, err); }
console.log(registration._id);
console.log(registration.id);
return res.json(201, registration);
});
};
Edit: this is the schema (I left out some fields that are not required):
'use strict';
var mongoose = require('mongoose'),
Schema = mongoose.Schema;
var RegistrationSchema = new Schema({
isReservation: Boolean,
//Id of the trip
tripId: String,
//socialMutuality
codeGerechtige: String,
socialMutualityNumberParent1: String,
socialMutualityNumberParent2: String,
//contact
userId: String,
firstnameContact: String,
lastnameContact: String,
emailContact: String,
streetContact: String,
streetNumberContact: String,
zipcodeContact: String,
busContact: String,
cityContact: String,
phoneContact: String,
gsmContact: String,
socialSecurityNumberContact: String,
//coordinats of person that pays
//child information
//emergency contacts
emergencyContacts: [{
firstName: String,
lastName: String,
phone: String
}],
extraInfo: String
});
module.exports = mongoose.model('Registration', RegistrationSchema);
Problem and solution: the problem was the client sending an attribute _id = null, and that's why MongoDB/Mongoose didn't update the id.

Answer 1

removing _id from req.body fixed my issue.
if(req.body._id === null) {
delete req.body._id;
}

Answer 2

There must be something else going on in your code which is effecting this. This example works as expected for me:
'use strict';
var mongoose = require('mongoose'),
Schema = mongoose.Schema;
mongoose.connect('mongodb://localhost/createid');
var RegistrationSchema = new Schema({
isReservation: Boolean,
//Id of the trip
tripId: String,
//socialMutuality
codeGerechtige: String,
socialMutualityNumberParent1: String,
socialMutualityNumberParent2: String,
//contact
userId: String,
firstnameContact: String,
lastnameContact: String,
emailContact: String,
streetContact: String,
streetNumberContact: String,
zipcodeContact: String,
busContact: String,
cityContact: String,
phoneContact: String,
gsmContact: String,
socialSecurityNumberContact: String,
//coordinats of person that pays
//child information
//emergency contacts
emergencyContacts: [{
firstName: String,
lastName: String,
phone: String
}],
extraInfo: String
});
var Registration = mongoose.model('Registration', RegistrationSchema);
var reg = {
userId: '1234',
tripId: '2345',
firstnameContact: 'Timothy',
lastnameContact: 'Strimple',
emailContact: 'tim#tstrimple.com'
};
Registration.create(reg, function(err, registration) {
if(err) { throw err; }
console.log(registration._id);
});
I get a valid id written out to the console.