title: Django ORM, sum of multiple columns
body: <p>I have a question about how we can filter by SUM of multiple columns.</p>
  
  <p>Example:</p>
  
  <pre><code>class Foo(models.Model):
      i1 = models.IntegerField()
      i2 = models.IntegerField()
      i3 = models.IntegerField()
  </code></pre>
  
  <p>And I need to filter objects where SUM of i1, i2, i3 is less then 200.
  I've tried achive it with:</p>
  
  <pre><code>Foo.objects.agregate(i_sum=Sum(i1,i2,i3)).filter(i_sum__lt=200) # error
  Foo.objects.agregate(i_sum=Sum([i1,i2,i3])).filter(i_sum__lt=200) # error
  </code></pre>
  
  <p>Thanks.</p>