'<' not supported between instances of 'method' and 'method' - Python, Django

Question

I'm trying to do Winerama Recommender Tutorial . I met a error which I can't solve. When I try to go to the tab 'recommendation list' the browser returned the following error.

Error

Environment:


Request Method: GET
Request URL: http://127.0.0.1:8000/recommendation/

Django Version: 2.0.7
Python Version: 3.7.0
Installed Applications:
['django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'bootstrap3',
 'reviews',
 'registration']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware']



Traceback:

File "C:\Users\tymot\AppData\Local\Programs\Python\Python37-32\lib\site-packages\django\core\handlers\exception.py" in inner
  35.             response = get_response(request)

File "C:\Users\tymot\AppData\Local\Programs\Python\Python37-32\lib\site-packages\django\core\handlers\base.py" in _get_response
  128.                 response = self.process_exception_by_middleware(e, request)

File "C:\Users\tymot\AppData\Local\Programs\Python\Python37-32\lib\site-packages\django\core\handlers\base.py" in _get_response
  126.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "C:\Users\tymot\AppData\Local\Programs\Python\Python37-32\lib\site-packages\django\contrib\auth\decorators.py" in _wrapped_view
  21.                 return view_func(request, *args, **kwargs)

File "C:\Users\tymot\Desktop\Cd-12.50-20.08\env\my_app\winerama\reviews\views.py" in user_recommendation_list
  89.         reverse=True

Exception Type: TypeError at /recommendation/
Exception Value: '<' not supported between instances of 'method' and 'method'

I created everything according to the steps in the tutorial.

File models.py

from django.db import models
from django.contrib.auth.models import User
import numpy as np


class Wine(models.Model):
    name = models.CharField(max_length=200)

    def average_rating(self):
        all_ratings = [list(map(lambda x: x.rating, self.review_set.all()))]
        return np.mean(all_ratings)

    def __unicode__(self):
        return self.name


class Review(models.Model):
    RATING_CHOICES = (
        (1, '1'),
        (2, '2'),
        (3, '3'),
        (4, '4'),
        (5, '5'),
    )
    wine = models.ForeignKey(Wine, on_delete=models.CASCADE)
    pub_date = models.DateTimeField('date published')
    user_name = models.CharField(max_length=100)
    comment = models.CharField(max_length=200)
    rating = models.IntegerField(choices=RATING_CHOICES)

class Cluster(models.Model):
    name = models.CharField(max_length=100)
    users = models.ManyToManyField(User)

    def get_members(self):
        return "\n".join([u.username for u in self.users.all()])

Next i added admin.py and create 3 klaster with users in '/admin'.

from django.contrib import admin

from .models import Wine, Review, Cluster

class ReviewAdmin(admin.ModelAdmin):
    model = Review
    list_display = ('wine', 'rating', 'user_name', 'comment', 'pub_date')
    list_filter = ['pub_date', 'user_name']
    search_fields = ['comment']


class ClusterAdmin(admin.ModelAdmin):
    model = Cluster
    list_display = ['name', 'get_members']


admin.site.register(Wine)
admin.site.register(Review, ReviewAdmin)
admin.site.register(Cluster, ClusterAdmin)

My file views.py

@login_required
def user_recommendation_list(request):
    # get request user reviewed wines
    user_reviews = Review.objects.filter(user_name=request.user.username).prefetch_related('wine')
    user_reviews_wine_ids = set(map(lambda x: x.wine.id, user_reviews))

    # get request user cluster name (just the first one righ now)
    user_cluster_name = \
        User.objects.get(username=request.user.username).cluster_set.first().name

    # get usernames for other memebers of the cluster
    user_cluster_other_members = \
        Cluster.objects.get(name=user_cluster_name).users \
            .exclude(username=request.user.username).all()
    other_members_usernames = set(map(lambda x: x.username, user_cluster_other_members))

    # get reviews by those users, excluding wines reviewed by the request user
    other_users_reviews = \
        Review.objects.filter(user_name__in=other_members_usernames) \
            .exclude(wine__id__in=user_reviews_wine_ids)
    other_users_reviews_wine_ids = set(map(lambda x: x.wine.id, other_users_reviews))

    # then get a wine list including the previous IDs, order by rating
    wine_list = sorted(
        list(Wine.objects.filter(id__in=other_users_reviews_wine_ids)),
        key=lambda x: x.average_rating,
        reverse=True
    )

    return render(
        request,
        'reviews/user_recommendation_list.html',
        {'username': request.user.username, 'wine_list': wine_list}
    )

I will mark that when I try use simple version everything works fine.

@login_required
def user_recommendation_list(request):
    # get this user reviews
    user_reviews = Review.objects.filter(user_name=request.user.username).prefetch_related('wine')
    # from the reviews, get a set of wine IDs
    user_reviews_wine_ids = set(map(lambda x: x.wine.id, user_reviews))
    # then get a wine list excluding the previous IDs
    wine_list = Wine.objects.exclude(id__in=user_reviews_wine_ids)

    return render(
        request,
        'reviews/user_recommendation_list.html',
        {'username': request.user.username,'wine_list': wine_list}
    )

My error is at this stage (2.5) of the tutorial. Stage 2.4 works good. Everything indicates that something is wrong in views.py.

Any help will be very appreciated.

Answer 1

The sorted() function accepts function key that returns value. Seems like x.average_rating is method, not the value. So you have 2 choices

• add () after x.average_rating
• convert x.average_rating to property

Answer 2

There is a difference between a method and the value a method returns. Here in your Wine model, we see:

from django.db.models import 

class Wine(models.Model):
    name = models.CharField(max_length=200)

    :
        return self.review_set.aggregate(
            mean=
        )['mean']

    def __unicode__(self):
        return self.name

(I rewrote it to make an efficienty query, instead of Python/Django/Numpy let do the work).

If you now have a Wine object named some_wine , then you do not get the average rating here with some_wine.average_rating , since this will return a method, but by calling the function, so some_wine.average_rating . 。

There are some options here:

1. calling the function in the lambda expression :

    wine_list = sorted( list(Wine.objects.filter(id__in=other_users_reviews_wine_ids)), key=lambda x: x.average_rating  , reverse=True ) 

2. define the method as a property, in that case, you no longer call the function, it is the called behind the curtains, so:

    from django.db.models import  class Wine(models.Model): name = models.CharField(max_length=200)  def average_rating(self): return self.review_set.aggregate( mean=  )['mean'] def __unicode__(self): return self.name 

3. use Wine.average_rating as key, since then it is a function that is called with instances:

    wine_list = sorted( list(Wine.objects.filter(id__in=other_users_reviews_wine_ids)), key=  , reverse=True ) 

4. order the Wine objects already by the database, by using .annotate(..) :

    wine_list = Wine.objects.filter( id__in=other_users_reviews_wine_ids ).annotate( mean=Avg('rating') ).order_by('-rating') 

The latest approach is probably the most efficient, since a database typically is optimized for such queries, furthermore this will get done by a single query.