我有一个客户数据库与交易。每个事务有2个sql列，即debit和credit。两者都可以填写，也可以只填写一个。
用雄辩，我想要的是借方和贷方的总和，这样我就可以得到他的余额。
事务架构如下所示：

Schema::create('transactions', function (Blueprint $table) {
          $table->increments('id');
          $table->datetime('datestamp')->nullable();;
          $table->text('reason')->nullable();;
          $table->decimal('debit')->default(0);
          $table->decimal('credit')->default(0);

          $table->integer('customerId')->unsigned();
          $table->foreign('customerId')->references('id')->on('customers');

          $table->boolean('isDeleted')->default(0);
          $table->timestamps();
      });

我的客户模型如下所示：

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class customer extends Model
{
    public function transaction()
    {
      return $this->hasMany('App\Models\transaction','customerId','id')->where("isDeleted",0)->orderBy('datestamp');
    }

}

我的事务模型如下所示：

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class transaction extends Model
{
    public function balance()
    {
        // problem is that i dont know how to get only for specific customer
        $debit = transaction::sum('debit');
        $credit = transaction::sum('credit');
        return $debit-credit; 
    }
}