title: Django filter a ForeignKey field when it is null
body: <p>Let's say I have two tables in Django, <code>TableA</code> and <code>TableB</code>. Table A contains some boolean field, <code>bool</code>, and <code>TableB</code> contains a foreign key field, <code>for_field</code> to <code>TableA</code>, which can be Null.</p>
  
  <pre><code>class TableA(models.Model):
      bool = models.BooleanField()
  
  class TableB(models.Model):
      for_field = models.ForeignKey('TableA', null=True)
  </code></pre>
  
  <p>If I want to filter TableB so as to get all the entries where <code>for_field.bool</code> is <code>True</code> or <code>for_field</code> is <code>Null</code>, what is the shortest way to achieve this?</p>
  
  <p>I'm using <code>.filter((Q(for_field__is_null=True) | Q(for_field__bool=True))</code>, but I wonder if there's shorter code for this.</p>