title: Django QuerySet: filter by the value of another field
body: <p>I have a model I wish to filter by its attribute <code>a</code>. The model also has another attribute <code>b</code>. I am trying to filter entries where <code>a</code> is 0 or <code>a</code> has the value of the <code>b</code> attribute (for each row, obviously). How can I filter by the value of another column?</p>
  
  <p>Here is what I have tried, and the missing piece:</p>
  
  <pre><code>MyModel.objects.filter(Q(a=0) | Q(a=???)) # ??? is to be the value of the `b` column
  </code></pre>
  
  <p>I am using Django 1.4 and I know it is an old version that is no longer supported but unfortunately performing the upgrade is not up to me.</p>