Need to save foreign key manually on belongsTo relationship with withDefault()? - laravel

Question

it seems that I need to save the original model on the newly generated model in order to save the relation correctly.
I hope the below test case illustrates my problem:
// migrations
Schema::create('users', function (Blueprint $table) {
$table->id();
$table->string('name');
$table->timestamps();
});
Schema::create('posts', function (Blueprint $table) {
$table->id();
$table->bigInteger('user_id')->nullable();
$table->timestamps();
});
//models
class User extends Model
{
public function posts()
{
return $this->hasMany(Post::class);
}
}
class Post extends Model
{
public function user()
{
return $this->belongsTo(User::class)
->withDefault(['name' => 'Guest User']);
}
}
//test
public function test_foreign_key()
{
$post = factory(Post::class)->create();
$user = $post->user; // created from ->withDefault()
$user->save();
$this->assertNotNull($post->user);
$this->assertNotNull($post->user_id);
}
With additional association of the original model the test passes:
public function test_foreign_key_associates()
{
$post = factory(Post::class)->create();
$post->user->save();
$this->assertNotNull($post->user);
$post->user()->associate($post);
$this->assertNotNull($post->user_id);
}
edit: what I'm doing right now is the following for the withDefault relationship:
class Post extends Model
{
public function user()
{
return $this->belongsTo(User::class)
->withDefault(function (User $user, Post $post) {
$user->name = "Guest User";
$user->save();
$post->user_id = $user->id;
});
}
}