title: How to update with function in laravel query builder
body: <p>I'm doing a query and want to update the data, but I want to do a math operation first. How to simplify it?</p>
  <p>my code below:</p>
  <pre><code>$jumlah = 10;
  $Rpodetail = Rpodetail::where('rpo_id',$request-&gt;postore)-&gt;where('produk_id',$produkid)-&gt;first();
  $total = $Rpodetail-&gt;sisa - $jumlah;
  Rpodetail::where('rpo_id',$request-&gt;postore)-&gt;where('produk_id',$produkid)-&gt;update([
  'sisa' =&gt; $total,
  ]);
                          
  </code></pre>