Histogram from django models

gistfile1.py

##### Assessor.models #####

class Question(models.Model):
    text = models.TextField()
    explanation = models.TextField()

class Choice(models.Model):
    text = models.TextField()
    is_correct = models.BooleanField(default=False)
    question = models.ForeignKey(Question)


class Answer(models.Model):
    user = models.ForeignKey(User)
    question = models.ForeignKey(Question)
    choice = models.ForeignKey(Choice)

##### Assessor.api.py #####

from django.contrib.auth.models import User
from Assessor.models import Answer, Choice, Question
from tastypie import Stuff

...
class AnswerHistogramResource(<WHAT HERE>)
    def generate_histogram(self, request):
        un = request.META['HTTP_X_USERNAME']
        user = User.objects.get_by_natural_key(un)
        answers = Answer.objects.filter(user=user)
        histogram = {}
        for a in answers:
            if not a.question_id in histogram.keys():
                histogram[a.question_id] = {'correct': 0, 'incorrect': 0}
            if Choice.objects.get(pk=a.choice_id).is_correct:
                histogram[a.question_id]['correct'] += 1
            else:
                histogram[a.question_id]['incorrect'] += 1
        return histogram