I need to restrict adding or deleting attachments in Purchase order when Purchase order is in Done state.It will getting error when i  inherit the create and unlink method of ir.attachment model.

from odoo import models, fields, api, _
from odoo.exceptions import UserError


class IrAttachment(models.Model):
    _inherit = 'ir.attachment'

    @api.model
    def create(self, vals):
        if vals.get('res_model') == 'purchase.order':
            po = self.env['purchase.order'].browse(vals.get('res_id'))
            if po.state == 'done':
                raise UserError('Can not add Attachments in Locked State')
            else:
                return super(IrAttachment, self).create(vals)

    @api.multi
    def unlink(self):
        if self.res_model == 'purchase.order':
            po = self.env['purchase.order'].browse(self.res_id)
            if po.state == 'done':
                raise UserError('Can not Delete Attachments in Locked State')
            else:
                return super(IrAttachment, self).unlink()

is there any altranative solution for this