title: Django Rest Framework Serializer popped off the validated_data after is_valid() call, causing KeyError in create
body: <p>I am just following the document to test to write a nested serializer from here</p>
  
  <p><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>Below is my testing code</p>
  
  <pre><code>    print(data_to_save)
      my_as = AlbumSerializer(data=data_to_save)
      my_as.is_valid()
      print(my_as.validated_data)
      my_as.save()
  </code></pre>
  
  <p>data_to_save is this</p>
  
  <pre><code>{'album_name': 'Peter ABC', 'artist': 'Peter', 'tracks': [OrderedDict([('order', 1), ('title', 'song 1'), ('duration', 11)]), 
  </code></pre>
  
  <p>OrderedDict([('order', 2), ('title', 'song 2'), ('duration', 11)])]}</p>
  
  <p>It has a key 'tracks'</p>
  
  <p>After calling is_valid(), the printed validated_data becomes</p>
  
  <pre><code>OrderedDict([('album_name', 'Peter ABC'), ('artist', 'Peter')])
  </code></pre>
  
  <p>Key 'tracks' kind of popped.</p>
  
  <p>It causes save() error</p>
  
  <pre><code>class AlbumSerializer(serializers.ModelSerializer):
      tracks = TrackSerializer(many=True, read_only=True)
  
      class Meta:
          model = Album
          fields = ('album_name', 'artist', 'tracks')
  
      def create(self, validated_data):
          print(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>tracks_data = validated_data.pop('tracks') will fail. The print(validated_data) inside create() also has 'tracks' key suddenly disappear</p>
  
  <pre><code>{'album_name': 'Peter ABC', 'artist': 'Peter'}
  </code></pre>
  
  <p>May I know why? My Django version 1.11.4 final and Django Rest Framework version is 3.6.3</p>