I am using odoo 10

I have a 2 models:

from odoo import models, fields, api

class BatchOrderLine(models.Model):

_name = 'batch.order.line'

product_id = fields.Many2one('product.product', string='Article')

description = fields.Text("Description")

class ProjectTask (models.Model):

_inherit = "project.task" 

   order_lines = fields.One2many('batch.order.line','batch_id',string="Articles")

@api.onchange('product_id') 

def product_id_change(self):

self.description = self.product_id

xml file :

<field name="product_id" on_change="product_id_change(product_id)"/>

 <field name="description" readonly="1"/>

My question is : how to update the "description" field value depending on the selected field "product_id" using onchange ?