title: Django rest serializer not serializing related fields
body: <p>In view set I am getting fields in request.data (passing foreign key) but serializer.data only gives non related fields
  I have 5 fields of which 1 is foreign key and 1 is manytomany relation.</p>
  
  <p>Model- </p>
  
  <p>class Movie(models.Model):</p>
  
  <pre><code>popularity = models.DecimalField(max_digits=3, decimal_places=1 )
  imdb_score = models.DecimalField(max_digits=2, decimal_places=1)
  name = models.CharField(max_length=200)
  director = models.ForeignKey(Director)
  genre = models.ManyToManyField(Genre)
  </code></pre>
  
  <p>Serializer - 
  class MovieSerializer(serializers.ModelSerializer):</p>
  
  <pre><code>class Meta:
      model = Movie
      fields='__all__'
      depth = 1
  </code></pre>
  
  <p>viweset - 
  class MovieViewSet(viewsets.ModelViewSet):</p>
  
  <pre><code>queryset = Movie.objects.all()
  serializer_class = MovieSerializer
  permission_classes = [IsAuthenticated]
  authentication_classes = [TokenAuthentication]
  </code></pre>