Laravel MongoDB associated model not setting foreign id - php

Question

I've set up a Laravel 9 & PHP 8 project and am using Mongo DB community version 5.0 as my NoSQL database. I've installed the jenssegers/laravel-mongodb package and have connected it to my MongoDB server.
I'm using MongoDB for the benefit of not having to define a schema for my migrations as such.
I've set up two models:
Application hasOne - application
ApplicationPayday - belongsTo application
Application model
<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Jenssegers\Mongodb\Eloquent\Model;
class Application extends Model
{
use HasFactory;
/**
* The collection associated with the model.
*
* #var string
*/
protected $collection = 'applications';
/**
* The attributes that aren't mass assignable.
*
* #var array
*/
protected $guarded = [
'id',
'created_at',
'updated_at'
];
/*
** Linked application payday
*/
public function payday()
{
return $this->hasOne(ApplicationPayday::class);
}
}
ApplicationPayday model
<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Jenssegers\Mongodb\Eloquent\Model;
class ApplicationPayday extends Model
{
use HasFactory;
/**
* The collection associated with the model.
*
* #var string
*/
protected $collection = 'application_paydays';
/**
* The attributes that aren't mass assignable.
*
* #var array
*/
protected $guarded = [
'id',
'application_id',
'created_at',
'updated_at'
];
/*
** Linked application
*/
public function application()
{
return $this->belongsTo(Application::class);
}
}
I have created two migrations for each of these models, with basic details, the rest of my data is saved without defining a specific migration which is what I need, however, since a ApplicationPayday is linked to an Application, I've created a foreginId field for this.
My migrations look like the following:
applications
Schema::create('applications', function (Blueprint $collection) {
$collection->id();
$collection->timestamps();
});
application_paydays
Schema::create('applications', function (Blueprint $collection) {
$collection->id();
$collection->foreignId('application_id');
$collection->timestamps();
});
Now, when I save my data, I want to automatically save associated models and link them together automatically by the fields, I tried this Stackoverflow, it's creating my data, but I don't see my application_id field in my ApplicationPayday model, why?
I cannot move it out of my $guarded array as the dat being saved is from a legacy system already containing the field, so of course, it wouldn't match up with the ids in MongoDB.
What am I missing from below:
// source data
$source = $request->input('application');
// create the application
$application = Application::create($source['Application']);
// payday application
$payday = ApplicationPayday::create($source['ApplicationPayday']);
$payday->application()->associate($application);