Room.php

    /**
     * filter only the bookings for the given date.
     * @see http://doctrine-orm.readthedocs.org/en/latest/reference/working-with-associations.html#filtering-collections
     */
    public function getBookingsOn(\DateTime $date)
    {
        $the_day       = new \DateTime($date->format('Y-m-d'));
        $the_day_after = $the_day->add(new \DateInterval('P1D'));

        $critera = Critera::create()
            ->where(Critera::expr()->gte('startAt', $the_day))
            ->andWhere(Critera::expr()->lt('startAt', $the_day_after))
            ->orderBy(['startAt' => Criteria::ASC]);

        $bookings = $this->getBookings()->matching($critera);
        return $bookings;
    }

SELECT * FROM bookings
INNER JOIN rooms ON rooms.id = bookings.room_id
WHERE DATE(bookings.start_at) = DATE(:date)