Laravel paginate polymorphic relationship, topic->comments() - php

Question

I'm building a basic forum, there are many topics and each topic has comments, a comment or "commentable" can be a a comment or a reply (commented a comment), this is because I want to have nested comments, that's where parent_id is useful. I'm wondering if my polymorphic relationship is the best way to achieve this so I'm open to any suggestions. The first issue I came up with is I don't know hw to do ajax pagination of topic->comments ..., I thought with something like Comment::whereHas('topic') ... but my comments dont' have a topic relationship defined in the model.
I'm using laravel advanced ajax pagination, how can I paginate comments?
Topic model:
class Topic extends Model
{
protected $table = 'topics';
public function author()
{
return $this->belongsTo('App\Models\User', 'user_id');
}
public function comments()
{
return $this->morphMany('App\Models\Comment', 'commentable')->whereNull('parent_id');
}
public function up()
{
Schema::create('topics', function (Blueprint $table) {
$table->increments('id');
$table->unsignedInteger('user_id')->index();
$table->foreign('user_id')->references('id')->on('users');
$table->string('title');
$table->text('body');
$table->string('url')->unique();
$table->string('slug')->unique();
$table->boolean('isVisible')->default(false);
$table->timestamps();
});
}
}
Comment model:
class Comment extends Model
{
protected $table = 'comments';
public function author()
{
return $this->belongsTo('App\Models\User', 'user_id');
}
public function replies()
{
return $this->hasMany('App\Models\Comment', 'parent_id');
}
}
public function up()
{
Schema::create('comments', function (Blueprint $table) {
$table->bigIncrements('id');
$table->integer('user_id')->unsigned();
$table->integer('parent_id')->unsigned();
$table->integer('commentable_id')->unsigned();
$table->string('commentable_type');
$table->text('body');
$table->timestamps();
});
}
This is where I took inspiration: