title: Modifying value on serialization - Django Rest Framework
body: <p>I have a model which contains sensitive data, let's say a social security number, I would like to transform that data on serialization to display only the last four digits.</p>
  
  <p>I have the full social security number stored: <code>123-45-6789</code>.
  I want my serializer output to contain: <code>***-**-6789</code></p>
  
  <p>My model:</p>
  
  <pre><code>class Employee (models.Model):
  
      name  = models.CharField(max_length=64,null=True,blank=True)
      ssn   = models.CharField(max_length=16,null=True,blank=True)
  </code></pre>
  
  <p>My serializer:</p>
  
  <pre><code>class EmployeeSerializer(serializers.ModelSerializer):
  
      id = serializers.ReadOnlyField()
  
      class Meta:
          model = Employee
  
          fields = ('id','ssn')
  
          read_only_fields = ['id']
  </code></pre>