title: If I change a field value in an api.onchange method, it's not updated in the view. Why?
body: <p>I have a function executing in a <code>api.onchange('bom_line_ids')</code>, inside this function I want to have a if statement to clear bom_line_ids if a condition is matched. I used <code>self.bom_line_ids.product_id = False</code></p>
  
  <p>but for some reason the field is cleared (saw this in the prints, but it's not updated in the view). </p>
  
  <pre><code>@api.onchange('bom_line_ids')
  def get_excl_list(self):
  
       list1 = []
       for i in self.excl_list:
           list1.append(i.id)
  
       list2 = []
       for j in self.bom_line_ids:
           list2.append(j.product_id.id)
  
           if j.product_id.id in list1:
               warning_message = "Product: " + self.env['product.template'].search([('id','=',j.product_id.id)]).name + " can't be added to the BoM.\n Please select another product."
               print "self.bom_line_ids.product_id", self.bom_line_ids.product_id
               self.bom_line_ids.product_id = False
               print "self.bom_line_ids.product_id", self.bom_line_ids.product_id
  
                return { 'warning': {'title': 'Product error', 'message':warning_message} }
  </code></pre>