Doctrine delete the oldest entry - php

Question

I have one MongoDb document with 3 fields message, author, sendAt, i have created a query builder to count all DB entries on this document and if there is more than 20 entries, I'm trying to delete the oldest one.
sendAt is the current dateTime when the message is send ("sendAt" : ISODate("2016-01-21T08:53:00Z"))
I have an unfinished query builder
public function deleteOldestMessage()
{
return $this->createQueryBuilder()
->delete('m')
->from('Messages', 'm')
->where('m.sendAt = ')
->execute();
}
But i really don't know what i should add on the where condition.
I should maybe order DESC the sendAt field and delete the first on the list ?
How can I tell the query builder to delete the oldest one ?
Thanks,

Answer 1

You need first to select the oldest one then you can delete it like this :
$qb->select('m')
->from('Messages', 'm')
->orderBy('m.sendAt', 'DESC');
$messages = $qb->getResult();
thene
if(count($messages) >= 20){
//get the first element of array which is the oldest one
$lastMessage = current($messages);
$em->remove($lastMessage);
}

Answer 2

You can try something like:
$qb = $em->createQueryBuilder();
$messages = $qb->select('m')
->from('Messages', 'm')
->orderBy('m.sendAt', 'ASC')
->getQuery()
->getResult()
;
// You will receive array where in the top will be the oldest message.
if (count($messages) > 20) {
// And in your desired case, you can remove just one oldest message.
$messageToDelete = array_shift($messages);
$em->remove($lastMessage);
$em->flush();
}

Answer 3

You use sql query builder options instead of mongodb.
return $this->createQueryBuilder()
->remove()
->sort('sendAt', 'asc')
->getQuery()
->getSingleResult()
->execute();