title: How to "group by" in django serializers for same model
body: <p>I am new to Django-Python and learning to build serializers. I understand the basic implementation of Serializers but stuck with this following particular requirement.</p>
  
  <p>This is my Customer Events model -</p>
  
  <pre><code>class CustomerEvents(models.Model):
         account_number = models.BigIntegerField(blank=False)
         event_type = models.CharField(max_length=255, blank=False, null=False)
  </code></pre>
  
  <p>Sample records in CustomerEvents:</p>
  
  <pre><code>Account Number    Event Type
  A12345             Billing Issues
  A12345             Video Services
  A12345             Sales
  </code></pre>
  
  <p>I want to create a CustomerEventsSerializer which will return values as below:</p>
  
  <pre><code>{ 
    "account_number" : A12345, 
    "event_types"    : ['Billing Issues', 'Video Services', 'Sales']
  }
  </code></pre>