Django how to make this view faster?

Question

I have view and it works correct, but very slow

class Reading(models.Model):
    meter = models.ForeignKey(Meter, verbose_name=_('meter'))
    reading = models.FloatField(verbose_name=_('reading'))
    code = models.ForeignKey(ReadingCode, verbose_name=_('code'))
    date = models.DateTimeField(verbose_name=_('date'))

    class Meta:
        get_latest_by = 'date'
        ordering = ['-date', ]

    def __unicode__(self):
        return u'%s' % (self.date,)

    @property
    def consumption(self):
        try:
            end = self.get_next_by_date(code=self.code, meter=self.meter)
            return (end.reading - self.reading) / (end.date - self.date).days
        except:
            return 0.0

    @property
    def middle_consumption(self):
        data = []
        current_year = self.date.year
        for year in range(current_year - 3, current_year):
            date = datetime.date(year, self.date.month, self.date.day)
            try:
                data.append(Reading.objects.get(
                    date = date,
                    meter = self.meter,
                    code = self.code
                ).consumption)
            except:
                data.append(0.0)
            for i in data:
                if not i:
                    data.pop(0)
        return sum(data) / len(data)

class DataForDayChart(TemplateView):
    def get(self, request, *args, **kwargs):
        output = []
        meter = Meter.objects.get(slug=kwargs['slug'])
        # TODO: Make it faster
        for reading in meter.readings_for_period().order_by('date'):
            output.append({
                "label": reading.date.strftime("%d.%m.%Y"),
                "reading": reading.reading,
                "value": reading.consumption / 1000,
                "middle": reading.middle_consumption / 1000
            })
        return HttpResponse(output, mimetype='application/json')

What should I change to make it faster?

Answer 1

There's nothing intrinsically time-consuming here code-wise - I think the best approach is to make sure that your database tables are set up correctly with the required indices. Perhaps create a view on the db to push some work onto that rather than select in code?

I am a little puzzled however by your middle_consumption function. The inner loop contents do something like:

get a date, x years ago from present day
get a reading on that date, or 0 on failure
add that reading to the results
go through the results
    if the current item is 0, delete the **first** item

This seems wrong.

1. What happens on February 29th? It would be better to add 365 days if that's appropriate for your application.

2. Why add a value only to (presumably want to) delete it again? Would something like this be better?

   try:
       current_value = Reading.objects.get(
           date = date_of_reading,
           meter = self.meter,
           code = self.code
       ).consumption
   except Reading.DoesNotExist: # or suitable exception
       current_value = 0
   if current_value > 0:
       anniversary_values.append( current_value )