title: Knex.js - How To Update a Field With An Expression
body: <p>How do we get Knex to create the following SQL statement:</p>
  
  <pre><code>UPDATE item SET qtyonhand = qtyonhand + 1 WHERE rowid = 8
  </code></pre>
  
  <p>We're currently using the following code:</p>
  
  <pre><code>knex('item')
      .transacting(trx)
      .update({qtyonhand: 10})
      .where('rowid', 8)
  </code></pre>
  
  <p>However, in order for our inventory application to work in a multi-user environment we need the qtyonhand value to add or subtract with what's actually in the database at that moment rather than passing a value that may be stale by the time the update statement is executed.</p>