Insert records according to key

def _insert(self, key, records, count):
        """ Insert records according to key """
        if key not in self.records:
            assert key not in self.counts
            self.records[key] = records
            self.counts[key] = count
        else:
            self.records[key] = np.vstack((self.records[key], records))
            assert key in self.counts
            self.counts[key] += count