Django Searchfilter not functioning I don't understand why

[mick]

views.py

class UserListView(generics.ListAPIView):
 queryset = User.objects.all()
 serializer_class = UserSerializer
 filter_backends = [filters.SearchFilter,OrderingFilter,]
 search_fields = ['first_name']

 def get(self,request):
    users=User.objects.all()
    serializer=UserSerializer(users,many=True)
    return Response(serializer.data)

 def post(self, request):
     serializer = UserSerializer(data=request.data)
     if serializer.is_valid():
         serializer.save()
         return Response(serializer.data, status=status.HTTP_201_CREATED)
     return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

 def delete(self, request, pk, format=None):
     user = self.get_object(pk)
     user.delete()
     return Response(status=status.HTTP_204_NO_CONTENT)

serializer.py

class UserSerializer(serializers.ModelSerializer):

    class Meta:
        model=User
        fields='__all__'


models.py

class User(models.Model):
  user_id =      models.AutoField(primary_key=True)
  first_name =   models.CharField(_("Firstname"),max_length=255,null = False)
  last_name =    models.CharField(_("last_name"),max_length=255,null = False)