title: how to find all many to many objects for parent of parent object in django
body: <p>I have the following object structure: </p>
  
  <pre><code>class Customer(models.Model):
      name = models.CharField('Customer Name', max_length=64)
  
  class Location(models.Model):
      customer = models.ForeignKey(Customer)
      groups = models.ManyToManyField(Group, blank=True, null=True)
  
  class Group(models.Model):
      name = models.CharField('Group Name', max_length=64)
  </code></pre>
  
  <p>How do I find all of the unique Group objects for a Customer?</p>