Custom Laravel Validation Rule

ExistsTopicInSubjectRule.php

<?php

namespace App\Rules;

use App\Models\Subject;
use Illuminate\Contracts\Validation\Rule;
use Illuminate\Http\Request;
use Illuminate\Support\Str;

class ExistsTopicInSubjectRule implements Rule
{
    private $request, $subject_key;

    /**
     * Create a new rule instance.
     *
     * @param Request $request Request instance
     * @param string $subjectKey Form subject key
     */
    public function __construct(Request $request, string $subjectKey)
    {
        $this->request = $request;
        $this->subject_key = $subjectKey;
    }

    /**
     * Determine if the validation rule passes.
     *
     * @param string $attribute
     * @param mixed $value
     * @return bool
     */
    public function passes($attribute, $value)
    {
        $subject = $this->findSubject($this->request, $attribute, $this->subject_key);

        if (!$subject) {
            return false;
        }

        $topic = $subject->topics()
            ->whereId($value)
            ->first();
        return $topic ? true : false;
    }

    /**
     * Get the validation error message.
     *
     * @return string
     */
    public function message()
    {
        return 'The selected :attribute is invalid.';
    }

    /**
     * Find Subject
     *
     * @param Request $request Request instance
     * @param string $attribute Current validation attribute
     * @param string $subject_key Subject key in request instance
     * @return Subject|null Subject model or null
     */
    private function findSubject(Request $request, string $attribute, string $subject_key): ?Subject
    {
        $subject_key = Str::of($attribute)->beforeLast('.')->append('.', $subject_key);
        $subject_id = $request->$subject_key;

        return Subject::find($subject_id);
    }
}