movies/app/Models/MediaListItem.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class MediaListItem extends Model
{
    protected $fillable = [
        'media_list_id',
        'item_id',
        'item_type',
        'tmdb_id',
    ];

    public function item()
    {
        if ($this->item_type == 'movie') {
            return $this->hasOne(Movie::class, 'id', 'item_id');
        }

        return $this->hasOne(Show::class, 'id', 'item_id');
    }
}