MongoDB updating empty field after initial insert - angularjs

Question

In the Mongoose schema there is an empty field for objectId of another table. I want to fill that field after the User object is created, as the school object is made after user object is.
User:
name
age
dob
company (this is object id from company table)
Initially I thought using the FindOneandUpdate() function would do this but I have had no luck.
Anyone any idea's?
company.component.ts
joinCompany(i){
console.log(this.companyArr[i]._id);
this.authService.addmoreUser(this.companyArr[i]._id).subscribe(data =>{
if(data.success){
this.router.navigate(['dashboard']);
}
else{
this.flashMessages.show(data.msg,{cssClass: 'alert-
danger',timeout:3000});
}
})
}
auth.service.js
addmoreUser(id){
id = {companyid:id};
console.log(id);
let headers = new Headers();
headers.append('Authorization', this.authToken);
headers.append('Content-Type','application/json');
return this.http.put('http://localhost:3000/users/pushCompanyintoUser',id,{headers:headers})
.map(res => res.json());
}
companies.js - CRUD file
router.put('/pushCompanyintoUser', passport.authenticate('jwt', {session:false}), (req, res, next) => {
console.log(req.body);
User.findByIdAndUpdate(req.user._id,{companyid:req.body} , function(err,user) {
if (err) throw res.json({success:false, msg:'Failed to Update user!'});
else res.json({success: true, msg:'User Updated!!'})
});
});
That is from the angular frontend being passed to server and then trying to insert to db.
DB Schema
const UserSchema = mongoose.Schema({
fullname:{ type: String},
email:{ type:String, required:true},
username:{ type:String, unique:true,required:true },
companyid:{ type:mongoose.Schema.Types.ObjectId, ref: 'Company', required:false}, <== (This is the where I am trying to submit to, in an object thats already made)
dob:{ type:String, required:true },
addressline:{ type:String, required:true },
town:{ type:String,required:true },
county:{ type:String,required:true},
country:{ type:String, required:true},
phone:{ type:Number, required:true },
password:{ type:String, required:true }
});
The User object is inserted already just without a companyid, how do I go about adding the companyid after.