from odoo import api, fields, models

class PurchaseOrder(models.Model):
    _inherit = "purchase.order"

    # create_uid must mirror the supervisor_id value.
    # create_uid is a magic field that belongs to the ORM that is not
    # editable via a form.
    create_uid = fields.Many2one(
        comodel_name='res.users',
        compute='_compute_create_uid',
    )
    supervisor_id = fields.Many2one(
        comodel_name='res.users',
        string='Responsible',
        required=True,
        default=lambda self: self.env.user,
    )

    @api.depends('supervisor_id')
    def _compute_create_uid(self):
        for rec in self:
            if rec.supervisor_id:
                rec.create_uid = rec.supervisor_id

    @api.multi
    def write(self, vals):
        if 'supervisor_id' in vals:
            new_supervisor = vals['supervisor_id']
            for rec in self:
                rec.message_unsubscribe_users(
                    user_ids=rec.supervisor_id.ids,
                )
                rec.message_subscribe_users(
                    user_ids=[new_supervisor],
                    subtype_ids=[],
                )
        return super(PurchaseOrder, self).write(vals)