hello 

for send  the notification you can do like that in your method.

        self.env['mail.message'].create({'message_type':"notification",
                "subtype": self.env.ref("mail.mt_comment").id, # subject type
                'body': "Message body",
                'subject': "Message subject",
                'needaction_partner_ids': [(4, self.user_id.partner_id.id)],   # partner to whom you send notification
                'model': self._name,
                'res_id': self.id,
                })