Autoincrement column with conditions - laravel

Question

I would like to implement an order column (that will be used for determining hierarchy for the entities, both in backend and frontend). Is there a nice way to do this on the migration level?
So that when a new model is saved, the highest number of the order column will be found - but only for the models that share shop_id. So if I add models like this:
// First model should get order = 1
$priceExceptionA = new PriceException();
$priceExceptionA->shop_id = 1;
$priceExceptionA->amount = 100;
$priceExceptionA->save();
// Second model should get order = 2
$priceExceptionB = new PriceException();
$priceExceptionB->shop_id = 1;
$priceExceptionB->amount = 100;
$priceExceptionB->save();
I would like to find a way to implement this behavior already through the migration:
public function up()
{
Schema::create('price_exceptions', function (Blueprint $table) {
$table->id();
$table->integer('shop_id')->unsigned();
$table->integer('amount')->nullable();
// Find the highest order value for rows with the same shop_id and increment this by one
$table->integer('order')->default(////)
$table->timestamps();
$table->softDeletes();
$table->foreign('shop_id')->references('id')->on('shops')->onDelete('cascade');
});
}

Answer 1

I do not believe SQL can what you want to do with the default functionality.
Instead there is a simple solution using model events, that secures all models will get these order values. The creating events, is called before the model is saved to the database and therefor all models will get the order set.
public static function boot()
{
static::creating(function(PriceException $priceException)
{
$priceException->order = ($priceException->shop->priceExceptions()->max('order') ?? 0) + 1;
});
}
This can create race conditions a way to combat this is to use a lock to secure you don't get duplicate order numbering. If you work on a load balanced setup the cache should be a shared redis server.
static::creating(function(PriceException $priceException)
{
Cache::lock($priceException->shop_id)->get(function () use ($priceException) {
$priceException->order = ($priceException->shop->priceExceptions()->max('order') ?? 0) + 1;
});
});