Bug in the tree view of the projects, when grouping by parent

Consider a hierarchical list of projects, with parent-child relation.
Try to group by "Parent" and you'll get an error such as:

Environment Information :
System : Windows-32bit
OS Name : nt
Operating System Release :
Operating System Version : 32bit
Operating System Architecture : 32bit
Operating System Locale : fr_CH.cp1252
Python Version : 2.5.2
OpenERP-Client Version : 6.0.0-rc2
Last revision No. & ID :Bazaar Package not Found !Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/openerp-server/netsvc.py", line 489, in dispatch
    result = ExportService.getService(service_name).dispatch(method, auth, params)
  File "/usr/local/lib/python2.6/dist-packages/openerp-server/service/web_services.py", line 598, in dispatch
    res = fn(db, uid, *params)
  File "/usr/local/lib/python2.6/dist-packages/openerp-server/osv/osv.py", line 122, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/usr/local/lib/python2.6/dist-packages/openerp-server/osv/osv.py", line 176, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/usr/local/lib/python2.6/dist-packages/openerp-server/osv/osv.py", line 167, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/usr/local/lib/python2.6/dist-packages/openerp-server/osv/orm.py", line 2182, in read_group
    cr.execute('SELECT min(%s.id) AS id, count(%s.id) AS %s_count' % (self._table, self._table, group_count) + (flist and ',') + flist + ' FROM ' + from_clause + where_clause + gb + limit_str + offset_str, where_clause_params)
  File "/usr/local/lib/python2.6/dist-packages/openerp-server/sql_db.py", line 76, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python2.6/dist-packages/openerp-server/sql_db.py", line 129, in execute
    res = self._obj.execute(query, params)
ProgrammingError: ERREUR: la référence à la colonne « parent_id » est ambigu
LIGNE 1 : ... id, count(project_project.id) AS parent_id_count,parent_id,...
                                                               ^

I think there is an error while constructing the query while grouping by parent: the colun 'parent_id' may be related to more than one table

Grouping by "Partner" and/or by "Manager" is okay.

Best regards