'Check all children object stores for the first one with the dataset.'

def _call_method(self, method, obj, default, default_is_exception, **kwargs):

    for (key, store) in self.backends.items():
        if store.exists(obj, **kwargs):
            return store.__getattribute__(method)(obj, **kwargs)
    if default_is_exception:
        raise default(('objectstore,    _call_method    failed:    %s    on    %s,    kwargs:    %s' % (method, str(obj), str(kwargs))))
    else:
        return default
