title: How do I override the results from an django API query
body: <p>I'm very new to Django, so this might seem basic. </p>
  
  <p>I have 2 models:</p>
  
  <pre><code>class Brand(models.Model):
      brand_name = models.CharField()
  
  class Foo(models.Model):
      brand = models.ForeignKey(Brand, null=True)
      foo_name = models.CharField()
  
  </code></pre>
  
  <p>Foo.brand can be null. Now when an API query is made on Foo, I want to include results where the brand's name matches but also where Foo.brand is null</p>
  
  <p>If Foo looks like this:</p>
  
  <pre><code>Foo.brand, Foo.foo_name
  1, "apple"
  3, "orange"
  1, "mango"
  null, "grape"
  </code></pre>
  
  <p>/api/foo?brand=1 should return apple, mango and grape. How can I get this? I tried reading similar questions, it sounds like I need to override the viewset, but wasn't sure. </p>
  
  <p>Thanks.</p>