title: django rest framework serializer add an auto generated field when getting data
body: <p>I want to automatically generate a field when data comes out of the database. Here is an example serializer I have:</p>
  <pre><code>class SaleSerializer(serializers.ModelSerializer):
      class Meta:
          model = Sale
          fields = '__all__'
  </code></pre>
  <p>I want this serializer to have two extra fields that dynamically generate one called username which has a string username of the user object and the other shop name that has the name of the shop object.</p>
  <p>Here is the model of sale that the serializer uses:</p>
  <pre><code>class Sale(models.Model):
      user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
      shop = models.ForeignKey(Shop, on_delete=models.DO_NOTHING)
      reciept = models.TextField()
      date_time = models.DateTimeField(auto_now_add=True)
      def __str__(self):
          return f&quot;reciept number {self.pk}&quot;
  </code></pre>