Symfony2 Doctrine trying to insert record twice? Race condition? - symfony

Question

We have a survey building application built in Symfony2 (think Survey Monkey or Google Forms). When a member of public accesses the survey, the various questions are built dynamically using Symfony's Form Builder and their answers are persisted using Doctrine. We currently collect an average of 1000 completed surveys a day without any trouble, but, every now and again, we get an Doctrine\DBAL exception.
Each answer to a question is persisted in a table which has a unique key consisting of the ID of the person filling in the survey and the ID of the question they are answering. Sometimes, when Doctrine tries to INSERT their answer, this key is violated and the query fails. As I mentioned, this happens fairly infrequently and we have been unable to replicate it on our test environment.
Things are made more complicated by the fact that all the Symfony Forms are build dynamically. Here is the code that actually does the persisting of the form data
public function save(\Symfony\Component\Form\Form $form, \Our\Namspace\Entity\Participant $participant)
{
/**
* $surveyReponse
*
* #var \Our\Namespace\Entity\SurveyResponse
*/
foreach ($form->getData() as $surveyResponseKey => $surveyResponse) {
$subQuestionId = $this->getQuestionIdFromSurveyResponseKey($surveyResponseKey);
$subQuestion = $this->getSubQuestionSettingsBySubQuestionId($subQuestionId);
if ($surveyResponse) {
$surveyResponse->setParticipant($participant)->setSubQuestion($subQuestion);
$participant->addResponse($surveyResponse);
$this->em->persist($surveyResponse);
}
$this->em->flush();
}
return true;
}
You can see that we encode the ID of the question (which is known as a SubQuestion in our domain) in the Form data in order to get the (sub)Question entity, then we set the Participant and the SubQuestion on the SurveyResponse object before persisting it. $this->em is just the Doctrine Entity Manager.
Without seeing all the code, it is probably difficult for anyone to figure out what is happening, but if anyone has had a similar problem in the past, maybe they could offer some advice.
We can and should wrap the persist and flush operations in a try/catch block and then handle the exception, but we'd really like to know why it is happening!
The relevant bits of the SurveyResponse entity look like
class SurveyResponse implements SurveyAwareInterface
{
/**
* id of the response.
*
* #var int
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="IDENTITY")
*/
protected $id;
/**
* the subQuestion to which the response refers.
*
* #var SurveySubQuestion
*
* #ORM\ManyToOne(targetEntity="\Our\Namspace\Entity\SurveySubQuestion", inversedBy="surveyResponses")
* #ORM\JoinColumn(name="sub_question_id", referencedColumnName="id")
*/
protected $subQuestion;
/**
* the participant.
*
* #var AbstractParticipant
*
* #ORM\ManyToOne(targetEntity="Our\Namespace\Entity\Participant", inversedBy="responses")
* #ORM\JoinColumn(name="participant_id", referencedColumnName="id", onDelete="CASCADE")
*/
protected $participant;
And there is this annotation in the Entity;
* #ORM\Table(name="survey_response",
* uniqueConstraints= #ORM\UniqueConstraint(
* name="participant_response", columns={"participant_id", "sub_question_id"})})
*