Using reverse relationships with django-rest-framework's serializer - django

Question

My model looks like this:
class User(TimestampedModel):
name = models.CharField(max_length=30, null=False, blank=False)
device = models.CharField(max_length=255, null=False, blank=False)
class Comment(TimestampedModel):
user = models.ForeignKey(User, on_delete=models.PROTECT, blank=True, null=True)
contents = models.CharField(max_length=510)
rating = models.IntegerField(blank=False, null=False)
And my serializer looks like this:
class UserSerializer(serializers.ModelSerializer):
class Meta:
model = User
fields = ('name',)
class CommentListItemSerializer(serializers.ModelSerializer):
user = UserSerializer()
class Meta:
model = Comment
fields = ('user', 'contents', 'rating')
And the view:
class CommentsList(generics.ListAPIView):
serializer_class = CommentListItemSerializer
queryset = Comment.objects.all()
It's almost getting the job done ;). The response I'm getting looks like this:
"results": [
{
"user": {
"name": "Ania"
},
"contents": "Very good",
"rating": 6
},
{
"user": {
"name": "Anuk"
},
"contents": "Not very good",
"rating": 1
}
]
There are two problems with that response.
I don't want to have this nested object "user.name". I'd like to receive that as a simple string field, for example "username".
Serializer makes a database query (not a join, but a separate query) for each user, to get his/her name. Since that's unacceptable, how to fix that?

Answer 1

Serializer makes a database query (not a join, but a separate query)
for each user, to get his/her name.
You can use select_related() on the queryset attribute of your view. Then accessing user.name will not result in further database queries.
class CommentsList(generics.ListAPIView):
serializer_class = CommentListItemSerializer
queryset = Comment.objects.all().select_related('user') # use select_related
I don't want to have this nested object "user.name". I'd like to
receive that as a simple string field, for example "username"
You can define a read-only username field in your serializer with source argument. This will return a username field in response.
class CommentListItemSerializer(serializers.ModelSerializer):
# define read-only username field
username = serializers.CharField(source='user.name', read_only=True)
class Meta:
model = Comment
fields = ('username', 'contents', 'rating')

Answer 2

You can add custom functions as fields
class Comment(models.Model):
user = models.ForeignKey(User, on_delete=models.PROTECT, blank=True, null=True)
contents = models.CharField(max_length=510)
rating = models.IntegerField(blank=False, null=False)
def username(self):
return self.user.name
class CommentListItemSerializer(serializers.ModelSerializer):
class Meta:
model = Comment
fields = ('username', 'contents', 'rating')