'return the action of the next screen ; this means the wizard is set to treat the
next wizard line. Each line is a subset of partner that can be merged together.
If no line left, the end screen will be displayed (but an action is still returned).'

@api.multi
def _action_next_screen(self):

    self.invalidate_cache()
    values = {}
    if self.line_ids:
        current_line = self.line_ids[0]
        current_partner_ids = literal_eval(current_line.aggr_ids)
        values.update({'current_line_id': current_line.id, 'partner_ids': [(6, 0, current_partner_ids)], 'dst_partner_id': self._get_ordered_partner(current_partner_ids)[(-1)].id, 'state': 'selection'})
    else:
        values.update({'current_line_id': False, 'partner_ids': [], 'state': 'finished'})
    self.write(values)
    return {'type': 'ir.actions.act_window', 'res_model': self._name, 'res_id': self.id, 'view_mode': 'form', 'target': 'new'}
