1. model

              customer_id = fields.Many2one('res.partner', string='Customer', required=True)
              customer_email = fields.Char()
              customer_mobile = fields.Char()


              @api.onchange('customer_id')
              def onchange_customer_id(self):
                  self.customer_email=self.customer_id.email
                  self.customer_mobile=self.customer_id.mobile