Unable to use Nova to create nested hierarchy of sub-sub-topics, Nova throws a 404 not found page

[connecteev]

I have a content_topics table, which contains a hierarchy of "content topics" (topics for short) as well as subtopics. Both are stored in the same content_topics table. A topic has a parent_id of NULL, wheras a sub-topic has a non-NULL parent_id.

My problem:

1. I am unable to use Nova to create sub-sub-topics, Nova throws a 404 not found page.
2. In case I would like to restrict / prevent sub-sub topics being created that are more than N levels deep (say more than 2 levels deep), how can I do that, either by making changes to my database schema, my Laravel Model and/or my Nova Resource?

Database table

Creating Top-level Topics works

I can create a top-level topic (with no parent) by going to /admin/resources/content-topics and clicking "Create Content Topic"

Creating SubTopic (under a Top-level Topic) works

I can create a 1st level sub-topic (with a non-null parent) by going to a top-level content topic like /admin/resources/content-topics/18 and clicking "Create Content Topic"

Creating Sub-SubTopic (under a SubTopic) Fails and results in a 404 page

However, if I try to nest this hierarchy further by going to a content sub-topic like /admin/resources/content-topics/25 and then try to click on "Create Content Topic", I see a 404 not found page.

My Code:

app/Models/ContentTopic.php:

<?php

namespace App\Models;

use App\Models\CourseContent\Course;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsTo;

class ContentTopic extends Model
{
    use HasFactory;

    public $timestamps = false;

    public function parent(): BelongsTo
    {
        return $this->belongsTo(ContentTopic::class, 'parent_id');
    }

    public function contentSubTopics()
    {
        return $this->hasMany(ContentTopic::class, 'parent_id', 'id');
    }

}

app/Nova/ContentTopic.php:

<?php

namespace App\Nova;

use Illuminate\Contracts\Database\Eloquent\Builder;
use Illuminate\Http\Request;
use Illuminate\Validation\Rule;
use Laravel\Nova\Fields\BelongsTo;
use Laravel\Nova\Fields\HasMany;
use Laravel\Nova\Fields\ID;
use Laravel\Nova\Fields\Text;
use Laravel\Nova\Http\Requests\NovaRequest;

class ContentTopic extends Resource
{
    /**
     * The model the resource corresponds to.
     *
     * @var class-string<\App\Models\ContentTopic>
     */
    public static $model = \App\Models\ContentTopic::class;

    /**
     * The single value that should be used to represent the resource when being displayed.
     *
     * @var string
     */
    public static $title = 'name';

    /**
     * The columns that should be searched.
     *
     * @var array
     */
    public static $search = [
        'id',
        'name',
        'parent.name',
    ];

    /**
     * Support eager database loading, so we also load the parent.
     * All relationships defined in $with will also be loaded whenever this resource is loaded.
     * Useful when we always want access to a resource in Nova (without making lazy database calls)
     * See https://laracasts.com/series/laravel-nova-mastery-2023-edition/episodes/8 at 12:00
     */
    public static $with = ['parent'];

    /**
     * Get the fields displayed by the resource.
     *
     * @param  \Laravel\Nova\Http\Requests\NovaRequest  $request
     * @return array
     */
    public function fields(NovaRequest $request)
    {
        return [
            ID::make()
                ->showWhenPeeking()
                ->sortable()
            ,

            Text::make('Name')
                ->showWhenPeeking()
                ->rules('required', 'string', 'max:255')
            ,

            BelongsTo::make('Parent', resource: self::class)
                ->nullable()
                ->relatableQueryUsing(fn ($request, $query) => $query->whereNull('parent_id'))
                ->showWhenPeeking()
                ->help('Specifying a parent indicates that this is a content subtopic')
                ->rules('nullable', Rule::exists('content_topics', 'id')->whereNull('parent_id'))
            ,

            HasMany::make('Content Sub-Topics', 'contentSubTopics', resource: ContentTopic::class)
            ,

        ];
    }
...
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