def identity(obj):
    """
    returns a string representing "<pk>,<version>" of the passed object
    """
    if hasattr(obj, '_concurrencymeta'):
        return mark_safe("{0},{1}".format(unlocalize(obj.pk),
                                          get_revision_of_object(obj)))
    else:
        return mark_safe(unlocalize(obj.pk))
returns a string representing "<pk>,<version>" of the passed object