Cannot resolve keyword 'tag' into field

Question

I am following the book 'Django 2 by example', and when it comes to implementing the tagging system for blog posts I encounter this error:

the error is caused when I click on the first-post tag shown here

here is models.py

from django.contrib.auth.models import User
from django.db import models
from django.utils import timezone
from django.urls import reverse
from taggit.managers import TaggableManager


class PublishManager(models.Manager):
    def get_queryset(self):
        return super(PublishManager, self).get_queryset().filter(status='published')


class Post(models.Model):
    STATUS_CHOICES = (
        ('draft', 'Draft'),
        ('published', 'Published'),
    )
    title = models.CharField(max_length=250)
    slug = models.SlugField(max_length=250, unique_for_date='publish')
    author = models.ForeignKey(User, on_delete=models.CASCADE, related_name='blog_post')
    body = models.TextField()
    publish = models.DateTimeField(default=timezone.now)
    created = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)
    status = models.CharField(max_length=10, choices=STATUS_CHOICES, default='draft')
    object = models.Manager()  # The default manager.
    published = PublishManager()  # My custom manager
    tags = TaggableManager()  # Imported from the library 'taggit'

    class Meta:
        ordering = ('-publish',)

    def __str__(self):
        return self.title

    def get_absolute_url(self):
        return reverse(
            'blog:post_detail',
            args=[
                self.publish.year,
                self.publish.month,
                self.publish.day,
                self.slug,
            ]
        )


class Comment(models.Model):
    post = models.ForeignKey(Post, on_delete=models.CASCADE, related_name='comments')
    name = models.CharField(max_length=80)
    email = models.EmailField()
    body = models.TextField()
    created = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)
    active = models.BooleanField(default=True)

    class Meta:
        ordering = ('created',)

    def __str__(self):
        return 'Comment by {} on {}'.format(self.name, self.post)

list.html

{% extends 'base.html' %}
{% block content %}
<h1>My Blog</h1>
{% if tag %}
<h2>Posts tagged with "{{ tag.name }}"</h2>
{% endif %}
{% for post in posts %}
<h2>
    <a href="{{ post.get_absolute_url }}">
        {{ post.title }}
    </a>
</h2>
<p class="tags">
    Tags:
    {% for tag in post.tags.all %}
    <a href="{% url 'blog:post_list_by_tag' tag.slug %}">
        {{ tag.name }}
    </a>
    {% if not forloop.last %}, {% endif %}
    {% endfor %}
</p>
<p class="date">
    Published {{ post.publish }} by {{ post.author }}
</p>
{{ post.body|truncatewords:30|linebreaks }}
{% endfor %}
<!--{% include "pagination.html" with page=page_obj %}-->
<!-- ^^^^^^^^^^^^ if using class-based view page=page_obj ^^^^^^^^^^ -->
{% include "pagination.html" with page=posts %}
<!-- ^^^^^^^^^^^^ if using function-based view page=posts ^^^^^^^^^^ -->
{% endblock %}

and urls.py

from django.urls import path
from . import views

app_name = 'blog'

urlpatterns = [
    path('', views.post_list, name='post_list'),
    # path('', views.PostListView.as_view(), name='post_list'),
    path('<int:year>/<int:month>/<int:day>/<slug:post>/', views.post_detail, name='post_detail'),
    path('<int:post_id>/share/', views.post_share, name='post_share'),
    path('tag/?<slug:tag_slug>/', views.post_list, name='post_list_by_tag'),
]

views.py

from django.core.paginator import Paginator, PageNotAnInteger, EmptyPage
from django.shortcuts import render, get_object_or_404
from .models import Post
from django.views.generic import ListView
from .forms import EmailPostForm, CommentForm
from django.core.mail import send_mail
from taggit.models import Tag


def post_list(request, tag_slug=None):
    object_list = Post.published.all()
    tag = None
    if tag_slug:
        tag = get_object_or_404(Tag, slug=tag_slug)
        object_list = object_list.filter(tag__in=[tag])
    paginator = Paginator(object_list, 3)
    page = request.GET.get('page')
    try:
        posts = paginator.page(page)
    except PageNotAnInteger:
        # if page is not an integer deliver the first page
        posts = paginator.page(1)
    except EmptyPage:
        # if page is out of range deliver last page of results
        posts = paginator.page(paginator.num_pages)
    return render(request, 'blog/post/list.html', {
        'posts': posts,
        'page': page,
        'tag': tag,
                                                   })

# ^^^^^^^^^^^^^^^^^^  SAME AS ^^^^^^^^^^^^^^^^^^^^^^^^ #


# class PostListView(ListView):
#     queryset = Post.published.all()
#     context_object_name = 'posts'
#     paginate_by = 3
#     template_name = 'blog/post/list.html'


def post_detail(request, year, month, day, post):
    post = get_object_or_404(Post,
                             slug=post,
                             status='published',
                             publish__year=year,
                             publish__month=month,
                             publish__day=day)
    # List of active comments for this post
    comments = post.comments.filter(active=True)
    new_comment = None
    if request.method == 'POST':
        # A comment was posted
        comment_form = CommentForm(data=request.POST)
        if comment_form.is_valid():
            # Create Comment objects but don't save to database yet
            new_comment = comment_form.save(commit=False)
            # Assign the current post to the comment
            new_comment.post = post
            # Save
            new_comment.save()
    else:
        comment_form = CommentForm()
    return render(request, 'blog/post/detail.html', {'post': post,
                                                     'comments': comments,
                                                     'new_comment': new_comment,
                                                     'comment_form': comment_form})


def post_share(request, post_id):
    # Retrieve post by id
    post = get_object_or_404(Post,
                             id=post_id,
                             status='published')
    sent = False
    if request.method == 'POST':
        # Form was submitted
        form = EmailPostForm(request.POST)
        if form.is_valid():
            # Form fields passed validation
            cd = form.cleaned_data
            post_url = request.build_absolute_uri(post.get_absolute_url())
            subject = '{} ({}) recommends you reading "{}"'.format(cd['name'], cd['email'], post.title)
            message = 'Read "{}" at {}\n\n{}\'s comments: {}'.format(post.title, post_url, cd['name'], cd['comments'])
            send_mail(subject, message, 'admin@mayblog.com', [cd['to']])
            sent = True
    else:
        form = EmailPostForm()
    return render(request, 'blog/post/share.html', {'post': post, 'form': form, 'sent': sent})

If you need any other code uploaded let me know about it. Hopefully, it's some easy-to-fix error.

Thanks for any help.

Answer 1

At post_list view, you need to change object_list = object_list.filter(tag__in=[tag]) to

   object_list = object_list.filter(tags__in=[tag])