دریافت نام دسته بندی بصورت نال در خروجی جیسون با جنگو

سلام.دوستان چرا نام دسته بندی من در خروجی جیسون که با رست فریمورک جنگو ساختم نال برمیگرده

from django.db import models
from django.contrib.auth.models import User
from django.utils import timezone

# Create your models here.

class Category(models.Model):
    parent = models.ForeignKey('self', default=None, null=True, blank=True, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    image = models.ImageField(upload_to='category', null=True)

    def __str__(self):
        return self.name

class Post(models.Model):
    author = models.ForeignKey(User, on_delete=models.CASCADE)
    title = models.CharField(max_length=200)
    body = models.TextField()
    status = models.BooleanField(default=False)
    image = models.ImageField(blank=True, default=True)
    favorite = models.BooleanField(default=False)
    published = models.DateTimeField(default=timezone.now)
    created = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)
    category = models.ManyToManyField(Category,)

    def __str__(self):
        return self.title

from .models import Post, Category
from rest_framework import serializers
from django.contrib.auth import get_user_model

class CategorySerializer(serializers.ModelSerializer):
    class Meta:
        model = Category
        fields = ["id", "name", "image"]

class AuthorSerializer(serializers.ModelSerializer):
    class Meta:
        model = get_user_model()
        fields = ["id", "username", "email"]

class PostSerializer(serializers.ModelSerializer):
    category = serializers.CharField(source="category.name", read_only=True)
    author = serializers.CharField(source="author.username", read_only=True)

    class Meta:
        model = Post
        fields = "__all__"

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = get_user_model()
        fields = "__all__"

from .models import Post, Category
from django.contrib.auth import get_user_model
from .serializers import PostSerializer, CategorySerializer, UserSerializer
from rest_framework.viewsets import ModelViewSet
from .permissions import IsStaffOrReadOnly, IsAuthorOrReadOnly, IsSuperUserOrStaffReadOnly

class UserView(ModelViewSet):
    queryset = get_user_model().objects.all()
    serializer_class = UserSerializer
    permission_classes = (IsSuperUserOrStaffReadOnly,)

class CategoryView(ModelViewSet):
    queryset = Category.objects.all()
    serializer_class = CategorySerializer

    def get_permissions(self):
        if self.action in ['list', 'create']:
            permission_classes = [IsStaffOrReadOnly]
        else:
            permission_classes = [IsStaffOrReadOnly, IsAuthorOrReadOnly]
        return [permission() for permission in permission_classes]

class PostView(ModelViewSet):
    queryset = Post.objects.all()
    serializer_class = PostSerializer
    filterset_fields = ['status', 'author__username']
    search_fields = ['title', 'body', 'author__username']
    ordering_fields = ['status', 'published']
    ordering = ['-published']

    def get_permissions(self):
        if self.action in ['list', 'create']:
            permission_classes = [IsStaffOrReadOnly]
        else:
            permission_classes = [IsStaffOrReadOnly, IsAuthorOrReadOnly]
        return [permission() for permission in permission_classes]