[Label] -> [(content_id, FeatureCollection)]

def content_objs_from_labels(self, labels):
        '''[Label] -> [(content_id, FeatureCollection)]'''
        is_mapping = lambda obj: isinstance(obj, collections.Mapping)
        def is_valid_fc((cid, fc)):
            if fc is None:
                return False
            if sum(1 for name in fc if is_mapping(fc[name])) == 0:
                return False
            return True

        ids = set()
        for lab in labels:
            ids.add(lab.content_id1)
            ids.add(lab.content_id2)
        return list(ifilter(is_valid_fc, self.store.get_many(ids)))