'Return message values of the current channel.
:param last_id : last message id to start the research
:param limit : maximum number of messages to fetch
:returns list of messages values
:rtype : list(dict)'

@api.multi
def channel_fetch_message(self, last_id=False, limit=20):

    self.ensure_one()
    domain = [('channel_ids', 'in', self.ids)]
    if last_id:
        domain.append(('id', '<', last_id))
    return self.env['mail.message'].message_fetch(domain=domain, limit=limit)
