Django 1.8 queryset上的条件注释

Question

I have a model that looks something like the following:

class MyStat(Model):
     metric = ForeignKey(MetricType) 
     source = ForeingKey(SourceType)

     total_units = IntegerField()
     metric_units = IntegerField()

     objects = MyQuerySet().as_manager()

The query does the following within a piece of its code

rollup = self.order_by().values(source.name)\.
                 annotate(mu=Sum('metric_units'), tu=Sum('total_units'))

And this works great. However, I would like to modify some of those annotation dependent on the metric type that is used. Somehow, create a database side conditional that will annotate slightly differently (pseudo code below):

if metric is some_metric:
      .annotate(mu=.8*Sum('metric_units'))
else:
      .annotate(mu=Sum('metric_units'))

Obviously the above syntax is invalid, but I'm wondering if there is a way to set up my queryset to do something like that on the database side

Answer 1

You can make conditional expresions with Case .

Let's go to the annotate part.

Disclaimer: This code is untested

from django.db.models import Case, Value, When, FloatField, Sum

...annotate(mu=Case(When(metric=some_metric, then=Value(.8*Sum('metric_units')),
                    default=Value(Sum('metric_units')),
                    output_field=FloatField())))

If you need to match against multiple values in a list, you can create a list of When objects:

whens = []
for some_metric in some_metrics:
    whens.append(When(metric=some_metric, then=Value(some-value-here)))

...annotate(mu=Case(*whens, default=Value(Sum('metric_units')),
                            output_field=FloatField()))