第一个问题是在这一行：

following_user = request.data.get('to_user')

序列化程序中没有这样的to_user字段，因此从请求中获取的值是None。请相应地替换该变量。

following_user = request.data.get('following_user')

其次，我们尝试直接使用ID而不是用户示例创建Follow对象，在尝试创建对象之前，只需获取用户示例。

from rest_framework import permissions
from core.models import Follow
from core.api.serializers import FollowSerializer
from django.contrib.auth import get_user_model

class FollowingView(APIView):
    permission_class = [permissions.IsAuthenticated]
    queryset = Follow.objects.all()
    serializer_class = FollowSerializer

    def post(self, request):
        user_id = request.data.get('user')
        following_user_id = request.data.get('following_user')

        user = get_object_or_404(get_user_model(), id=user_id)
        following_user = get_object_or_404(get_user_model(), id=following_user_id)

        try:
            follow = Follow.objects.create(
            user=user, following_user=following_user)
            follow.save()
            serializer = FollowSerializer(follow)
            print(serializer.data)

            def __str__(self):
                return f"{self.request.username} follows {self.following_user_id.username}"

            return Response(serializer.data, status=status.HTTP_201_CREATED)
        except:
            return Response(status=status.HTTP_400_BAD_REQUEST)

编辑：有关如何正确使用序列化程序验证数据，请参阅@Ankit Tiwari的回答

如果不从客户端获取数据，请首先将 read_only=True 设置为user&following_user字段。

class FollowSerializer(serializers.ModelSerializer):
    user = serializers.PrimaryKeyRelatedField(many=False, read_only=True)
    following_user = serializers.PrimaryKeyRelatedField(many=False, read_only=True)
    class Meta:
        model = Follow
        fields = ['user', 'following_user', 'date_followed']

在视图中，只需验证序列化程序并将其与相关字段一起保存

class FollowingView(APIView):
    permission_class = [permissions.IsAuthenticated]
    queryset = Follow.objects.all()
    serializer_class = FollowSerializer

    def post(self, request):
        user = request.data.get('user')
        following_user = request.data.get('to_user') # check if it's correct

        try:
            user = User.objects.get(id=user)
            following_user = User.objects.get(id=following_user)
            serializer = FollowSerializer(request.data)
            if serializer.is_valid():
               serializer.save(
                 user=user,
                 following_user=following_user
               )

            def __str__(self):
                return f"{self.request.username} follows {self.following_user_id.username}"

            return Response(serializer.data, status=status.HTTP_201_CREATED)
        except:
            return Response(status=status.HTTP_400_BAD_REQUEST)

您需要删除与object.create一起使用的.save()方法。
object.create用于保存表中的数据对象，因此无需.save()

follow = Follow.objects.create(user=user, following_user=following_user)
serializer = FollowSerializer(follow)
print(serializer.data)

您还可以保存数据并使用APIView()方法对其进行序列化，如下所示

def post(self, request):
    serializer = FollowSerializer(data=request.data)
    print(serializer.is_valid()) ## for debug
    print(serializer.errors) ## for debug
    if serializer.is_valid():
        serializer.save()
        return Response({'msg':'Data Created'}, status=status.HTTP_201_CREATED)
    return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)