osb/app/Traits/NextKey.php

<?php

/**
 * Works out the next ID to use for an Eloquent Table.
 *
 * If we update records, we update the record_id table
 */
namespace App\Traits;

use App\Models\{Module,Record};

trait NextKey
{
	public static function boot()
	{
		parent::boot();

		static::creating(function($model)
		{
			$model->id = self::NextId();

			if (! $model->site_id)
				$model->site_id = config('SITE_SETUP')->id;
		});

		static::saved(function($model)
		{
			if ($model->wasRecentlyCreated) {
				if (! defined(get_class($model).'::RECORD_ID'))
					throw new \Exception('Missing record_id const for '.get_class($model));

				$mo = Module::where('name',$model::RECORD_ID)->firstOrFail();

				if (! $mo->record) {
					$mo->record = new Record;
					$mo->record->module_id = $mo->id;
					$mo->record->site_id = 1; // @todo
				}

				$mo->record->id = $model->id;
				$mo->record->save();
			}
		});
	}

	public static function NextId()
	{
		return (new self)->max('id')+1;
	}
}