'Change the _id field in fs_entry into the properly ordered SON or string
Args:
fs_entry: the element returned by self.fs_files.find'

def make_id_son(self, fs_entry):

    _id_field = fs_entry.get('_id', fs_entry)
    if isinstance(_id_field, basestring):
        return _id_field
    dbkey = SON(((field_name, _id_field.get(field_name)) for field_name in self.ordered_key_fields))
    if ('run' in _id_field):
        dbkey['run'] = _id_field['run']
    fs_entry['_id'] = dbkey
    return dbkey
