title: Odoo onchange not working correctly
body: <p>I'm inherit purchase.order.line and try change value in field. For product_qty I can change value but for price_unit I can't change value.</p>
  
  <p>My custom .py file:</p>
  
  <pre><code>class PurchaseOrderLine(models.Model):
  
  _inherit = 'purchase.order.line'
  
  
  @api.onchange('product_id')
  def my_fucn(self):
      for rec in self:
          rec.product_qty = 10  #WORKING
          rec.price_unit = 1    #NOT WORKING
  </code></pre>
  
  <p>Maybe is problem because in original purcahase.py odoo file also have @api.onchange('product_id').</p>
  
  <p>Any solution?</p>