title: DjangoRestFramework - How to add relational field to serializer without overriding it?
body: <p>This is my model:</p>
  
  <pre><code>class UserExtended(models.Model):
      user = models.OneToOneField(User)
      message = models.CharField(max_length=400, blank=True, null=True)
  </code></pre>
  
  <p>It has a OneToOneField relationship with the default Django User model. This is my UserSerializer:</p>
  
  <pre><code>class UserSerializer(serializers.ModelSerializer):
  
      message = serializers.CharField(source='userextended.message')
  
      class Meta:
          model = User
          fields = ('username', 'password', 'email', 'message',)
  
          extra_kwargs = {
                      'password': {
                          'write_only': True,
                      },
                      'email': {
                          'required': True,
                      }
                  }
  </code></pre>
  
  <p>The problem with this is that now DRF thinks that the <code>message</code> field is required, even though I have <code>blank=True</code> and <code>null=True</code> in my model. </p>
  
  <p>1) How do I add the <code>message</code> field to the serializer without overriding it?</p>
  
  <p>OR</p>
  
  <p>2) Is there a way for me to check how the <code>message</code> field would look if DRF serialized it by default (i.e. if I did not override it / if UserExtended had it's own serializer)? That way when I do override it, I can make it the exact same as how DRF would originally make it if <code>UserExtended</code> had a serializer.</p>
  
  <p>What I mean by #2 is, generally DRF will automatically add <code>required=False</code> to the <code>message</code> serializer field (because <code>blank=True</code> and <code>null=True</code> in my model field) if I was serializing <code>UserExtended</code> (it might also automatically add something else as well which I do not know). However, because in my current situation (I cannot just add <code>message</code> to the list of fields and I can only do it by overriding it and using <code>source</code>), I don't know what else DRF would have automatically added other than the <code>required=False</code> to the serializer.</p>