title: DRF IntegrityError: NOT NULL constraint failed: user_id
body: <p>I can't figure out how to pass user object to the following serializer:</p>
  
  <pre><code>class ReviewSerializer(serializers.ModelSerializer):
      user = UserSerializer(read_only=True)
  
      class Meta:
          model = Review
          fields = ('pk', 'title', 'user', 'movie', 'timestamp', 'review_text',)
  </code></pre>
  
  <p>I have this viewset:</p>
  
  <pre><code>class ReviewsViewSet(viewsets.ModelViewSet):
      queryset = Review.objects.all()
      serializer_class = ReviewSerializer
  </code></pre>
  
  <p>and this model:</p>
  
  <pre><code>class Review(models.Model):
      title = models.CharField(max_length=255)
      user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='reviews')
      movie = models.ForeignKey(Movie, on_delete=models.CASCADE, related_name='reviews')
      review_text = models.TextField()
      timestamp = models.DateTimeField(auto_now_add=True)
  
      def __str__(self):
          return '{movie} review by {user}'.format(user=self.user, movie=self.movie)
  </code></pre>
  
  <p>My javascript request looks like this:</p>
  
  <pre><code>return axios({
    method: 'post',
    url: 'http://localhost:8000/api/reviews/',
    data: { // Using data from Vue
      title: this.review_title,
      movie: this.id,
      review_text: this.review_text,
      user: JSON.stringify(this.user)
    },
    headers: {
      'Content-Type': 'application/json',
      Authorization: `JWT ${token}`
    }
  })
  </code></pre>
  
  <p>It gives me <a href="http://dpaste.com/144C4KH" rel="nofollow noreferrer">this traceback</a>.</p>
  
  <p>How should I pass the user object to the request?</p>
  
  <p>Thanks in advance.</p>