title: Writable nested Django serializer with string instead of dict
body: <p>The Django Rest Framework docs have an example for writable nested serializers, see <a href="http://www.django-rest-framework.org/api-guide/relations/#writable-nested-serializers" rel="nofollow noreferrer">http://www.django-rest-framework.org/api-guide/relations/#writable-nested-serializers</a>.</p>
  
  <p>I am interested in the case where the related <code>Track</code> model has only one field (<code>title</code>) besides the foreign key field. I thus modified it to read:</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class Album(models.Model):
      album_name = models.CharField(max_length=100)
      artist = models.CharField(max_length=100)
  
  class Track(models.Model):
      album = models.ForeignKey(Album, related_name='tracks', on_delete=models.CASCADE)
      title = models.CharField(max_length=100)
  
      class Meta:
          unique_together = ('album', 'title')
  </code></pre>
  
  <p><strong>serializers.py</strong></p>
  
  <pre><code>class TrackSerializer(serializers.ModelSerializer):
      class Meta:
          model = Track
          fields = ('title',)
  
  class AlbumSerializer(serializers.ModelSerializer):
      tracks = TrackSerializer(many=True)
  
      class Meta:
          model = Album
          fields = ('album_name', 'artist', 'tracks')
  
      def create(self, validated_data):
          tracks_data = validated_data.pop('tracks')
          album = Album.objects.create(**validated_data)
          for track_data in tracks_data:
              Track.objects.create(album=album, **track_data)
          return album
  </code></pre>
  
  <p><strong>now run:</strong></p>
  
  <pre><code>&gt;&gt;&gt; data = {
      'album_name': 'The Grey Album',
      'artist': 'Danger Mouse',
      'tracks': [
          {'title': 'Public Service Announcement'},
          {'title': 'What More Can I Say'},
          {'title': 'Encore'},
      ],
  }
  &gt;&gt;&gt; serializer = AlbumSerializer(data=data)
  &gt;&gt;&gt; serializer.is_valid()
  True
  &gt;&gt;&gt; serializer.save()
  &lt;Album: Album object&gt;
  </code></pre>
  
  <p><strong>Question:</strong></p>
  
  <p>I need the tracks in the serialized data to be structured as a list of track title strings, instead of a list of dicts <code>{'title': title}</code>, like this:</p>
  
  <pre><code>&gt;&gt;&gt; data = {
      'album_name': 'The Grey Album',
      'artist': 'Danger Mouse',
      'tracks': [
          'Public Service Announcement',
          'What More Can I Say',
          'Encore',
      ],
  }
  </code></pre>
  
  <p>I have attempted adapting <code>AlbumSerializers</code>'s <code>to_representation</code> and <code>to_internal_value</code> methods, but could not get it to work with the requirement that the serializer be writable. What am I missing?</p>