title: Onchange triggered only once
body: <p>In sale.order I made the field and I want when SO state changes this field will fill up based on current UID</p>
  
  <p>the problem onchange is triggered only once when I create SO, but when I confirm it, ocnahnge is not triggered. </p>
  
  <p>What do I miss about onchange ?  </p>
  
  <pre class="lang-py prettyprint-override"><code>user_prepared = fields.Many2one('res.users', string="Prepared offer")
  
          @api.onchange('state')
          def _onchange_state(self):
               self.user_prepared = self._uid
  </code></pre>