Question

I am working on a questionnaire type app for Django.
When participants complete a questionnaire an AnswerSet object is created which links their user object to a list of QuestionAnswer object, one for each question that they answer.

class AnswerSet(models.Model):

    user=models.ForeignKey(User)
    questionnaire=models.ForeignKey(Questionnaire)


class QuestionAnswer(models.Model):    

    question = models.ForeignKey(Question)
    answer = models.CharField(max_length=255)
    answer_set = models.ForeignKey(AnswerSet)

The app allows people to re-answer a questionnaire, and in this case renders the form using their existing answers, and for answers that are updated, a new QuestionAnswer object is created and saved.

My question therefore is this:

In order to display the most recent answer when the participant is editing the questionnaire, I want to be able to get an AnswerSet, and then filter the QuestionAnswers so that I get a QuestionAnswer for each question, and where there are more than one QuestionAnswer for any question, I only want to see the most recent one

Answer 1

Thanks to the suggestion by @Rohan and this post Django query select distinct by field pairs I have found a solution that works for me, hopefully someone else will find it useful too!
I have added a new model:

class LatestQuestionAnswer(models.Model):
    question = models.ForeignKey(Question)
    question_answer = models.ForeignKey(QuestionAnswer)
    answer_set = models.ForeignKey(AnswerSet)
    created = created = models.DateTimeField(auto_now_add=True)

And overridden the save function on my QuestionAnswerModel so it looks like:

def save(self, force_insert=False, force_update=False, using=None):
        super(QuestionAnswer, self).save(force_insert=force_insert, force_update=force_update, using=using)

        try:
            record = LatestQuestionAnswer.objects.get(question=self.question, answer_set=self.answer_set)

            if record.question_answer == self:
                return#nothing to do no point updating the record as it is already correct

        except ObjectDoesNotExist:
            record = LatestQuestionAnswer(question=self.question, answer_set= self.answer_set)

        record.question_answer = self
        record.save()

Now whenever I need to get a list of all the most recent QuestionAnswers I can do something like this:

most_recent_answers = [record.question_answer for record in LatestQuestionAnswer.objects.filter(answer_set=<an_answer_set>)]