title: How to sum only positive values, with Django conditional aggregates?
body: <p>I have a table that looks like this:</p>
  
  <pre><code>class MeasureValue(models.Model):
      cost_saving_10th = models.FloatField(null=True, blank=True)
  </code></pre>
  
  <p>I know how to get the sum of all the values:</p>
  
  <pre><code>mvs.aggregate(Sum('cost_saving')).values()[0]
  </code></pre>
  
  <p>But how do I get the sum of all the <em>positive</em> values? I think I need a conditional aggregate, but I'm not sure how to do this. </p>