Django Rest Framework - "unique_together" internal server error. How to respond with error

Question

I currently have a model called "Review" where I have a "unique_together" constraint so one user can only write one review per post.

class Review(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    user = models.ForeignKey(User, related_name='reviews', on_delete=models.SET_NULL, null=True)
    post = models.ForeignKey(Post, related_name='reviews', on_delete=models.CASCADE)
    rating = models.IntegerField()
    comment = models.TextField(blank=True, null=True)

    class Meta:
        unique_together = ['user', 'post']

And in my View I am grabbing the user from the request (I'm using token authentication) and use that user to save the object by overwriting the "perform_create" method:

class ReviewCreateView(CreateAPIView):
    permission_classes = [IsAuthenticated]
    serializer_class = ReviewCreateSerializer

    def perform_create(self, serializer):
        serializer.save(user=self.request.user)

And my serializer looks like this:

class ReviewCreateSerializer(ModelSerializer):
    class Meta:
        model = Review
        fields = ['post', 'rating', 'comment']

When I try to submit two reviews to the same post by the same user I get an internal server error.

The body of my HTTP POST request looks like this:

{
    "post" : 11,
    "rating": 1,
    "comment": "this is a great post"
}

Instead of getting an error response (for example 403 error). I get this Django server error instead:

django.db.utils.IntegrityError: duplicate key value violates unique constraint

After spending some time researching I saw that Django Rest Framework provides the "UniqueTogetherValidator". So I added it like this:

class ReviewCreateSerializer(ModelSerializer):
    class Meta:
        model = Review
        fields = ['post', 'rating', 'comment']
        validators = [
                UniqueTogetherValidator(
                    queryset=Review.objects.all(),
                    fields=['user', 'post']
                )
            ]

But then the issue I'm having is that it errors because it can't find the key "user", I'm guessing because I dont have a "user" field in my HTTP request body (since the user is being grabbed by Django using the authentication token). I get the following error:

KeyError: 'user'

I also tried adding the 'user' to my serializer fields:

fields = ['user', 'post', 'rating', 'comment']

But I get the following error response:

{
    "user": [
        "This field is required."
    ]
}

I would really appreciate some help to solve this issue. Many thanks!