Maybe use something like this to wrap the relationship in an accessor. The nonExistantComments could be something like a JSON field in the database that holds the "not saved" data

Post.php

<?php

use Illuminate\Support\Collection;

class Post extends Model {
  
  private $nonExistantComments;
  
  public function __construct() {
    $this->nonExistantComments = collect([]);
  }
  
  public function comments() {
    return $this->hasMany(Comments::class);
  }
  
  public function getCommentsAttribute(Collection $value) {
    return $value->merge($this->nonExistantComments);
  }

}