Hello Clément,

this is exactly what does portal module :

`

try:
    self.pool.get(mail.model).check_access_rule(cr, related_user.id, [mail.res_id], 'read', context=context)

    url = partner_obj._get_signup_url_for_action(cr, related_user.id, [partner.id], action='', res_id=mail.res_id, model=mail.model, context=context)[partner.id]

    text = _("""<p>Access this document <a href="%s">directly in OpenERP</a></p>""") % url

except except_orm, e:

`

this code is from addons/portal/mail_mail.py.

regards, Marc.

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