For example, there is a sale order report and it's id = "action_report_saleorder",

consequently REPORT_ID in example = sale.action_report_saleorder

(Example odoo v11)

def action_generate_attachment(self):

        """ this method called from button action in view xml """

        # generate pdf from report, use report's id as reference

        REPORT_ID = 'some_report_id'

        pdf = self.env.ref(REPORT_ID).render_qweb_pdf(self.ids)

        # pdf result is a list

        b64_pdf = base64.b64encode(pdf[0]) 

# save pdf as attachment

        ATTACHMENT_NAME = "My Attachment Name"

        return self.env['ir.attachment'].create({

            'name': ATTACHMENT_NAME,

            'type': 'binary',

            'datas': b64_pdf,

            'datas_fname': ATTACHMENT_NAME + '.pdf',

            'store_fname': ATTACHMENT_NAME,

            'res_model': self._name,

            'res_id': self.id,

            'mimetype': 'application/x-pdf'

        })

Why when I perform a similar action, nothing works for me?
Can you explain what this method render_qweb_pdf( ) means and what is the difference between id and res_id?

class AttachSaleOrderFile(models.Model):
_inherit='sale.order'
@api.multi
def action_generate_attachment(self):

.....

.....

Thanks!