Holy hardcoded system, Batman!

EntityManager.php

    /**
     * {@inheritDoc}
     */
    public function newHydrator($hydrationMode)
    {
        switch ($hydrationMode) {
            case Query::HYDRATE_OBJECT:
                return new Internal\Hydration\ObjectHydrator($this);

            case Query::HYDRATE_ARRAY:
                return new Internal\Hydration\ArrayHydrator($this);

            case Query::HYDRATE_SCALAR:
                return new Internal\Hydration\ScalarHydrator($this);

            case Query::HYDRATE_SINGLE_SCALAR:
                return new Internal\Hydration\SingleScalarHydrator($this);

            case Query::HYDRATE_SIMPLEOBJECT:
                return new Internal\Hydration\SimpleObjectHydrator($this);

            default:
                if (($class = $this->config->getCustomHydrationMode($hydrationMode)) !== null) {
                    return new $class($this);
                }
        }

        throw ORMException::invalidHydrationMode($hydrationMode);
    }