title: Django rest framework - model serializer doesn't serialize all fields
body: <p>I'm using Django rest framework 3.7.7</p>
  
  <p>I want to serialize an object instance and then deserialize it (without saving to DB again).
  So I created a serializer that inherits from serializers.ModelSerializer.
  To this model instance I sometimes add a field within the request that is not part of the model, using property setter. I did add it to the serializer as well, but when serializing - I don't see that field in the serialized data.</p>
  
  <p>This is the model:</p>
  
  <pre><code>class MyModel(models.Model):
  
      manager = models.CharField(max_length=200)
      reviewer = models.CharField(max_length=200)
  
      @property
      def note(self):
          return getattr(self, '_note', None)
  
      @note.setter
      def note(self, value):
          self._note = value
  </code></pre>
  
  <p>This is the model serializer:</p>
  
  <pre><code>class MyModelSerializer(serializers.ModelSerializer):
      note = serializers.CharField(write_only=True, required=False, max_length=1000, allow_null=True)
  
      class Meta:
          model = MyModel
          fields = [
              'manager',
              'reviewer',
              'note'
          ]
  </code></pre>
  
  <p>This is how I use the serializer:</p>
  
  <pre><code>ser_instance = MyModelSerializer(self.instance)
  ser_data = ser_instance.data
  </code></pre>
  
  <p>seems like the <code>ser_instance.data</code> contains only the model fields data (without the 'note')</p>