public function addKey($table, $hash, $range = null)
    {
        if ($this->count() >= 100) {
            throw new \Riverline\DynamoDB\Exception\AttributesException("Can't request more than 100 items");
        }

        if (!isset($this->keysByTable[$table])) {
            $this->keysByTable[$table] = array();
        }

        $this->keysByTable[$table][] = array($hash, $range);

        return $this;
    }