Django urlpatterns when a category is included into URL - django

Question

Django 3.0.6
urlpatterns = [
path('', HomeView.as_view(), name='home'),
path('{}'.format("admin/" if DEBUG else "dhjfsljdasdhje32/"), admin.site.urls), # Change admin url for security reasons.
path('image/', include(('image.urls', 'image'), namespace="image")),
path('polls/', include(('polls.urls', 'polls'), namespace="polls")),
path('applications/', include(('applications.urls', 'applications'), namespace="applications")),
]
def _get_categories_url_pattern():
"""
Organize urls with posts categories.
URL format:
<category>/<post_slug>
Example: linux/how_to_install_ubuntu/
"""
categories = Category.objects.all().values_list("slug", flat=True)
for category in categories:
urlpatterns.append(path('{}/'.format(category), include('post.urls')))
urlpatterns.append(path('draft/{}/'.format(category), include('post.urls')))
_get_categories_url_pattern()
Please, concentrate your attention on how categories in urls are handled.
Problems with this code:
When a new category is added in the admin site, Django project has
to be relaunched (at least with the built in dev server).
When last time I did python manage.py makemigrations it blew up. I
had to comment out anything having anything to do with
_get_categories_url_pattern. Then it made migrations. Now I can't reproduce this error anymore. But there seems to be some danger in this code.
Could you help me understand how to refactor this code for it to work at least without restarting the dev server when a new category is added.

Answer 1

The urls are loaded when the server starts, so you should avoid doing database queries there.
When you run migrate for a fresh database, your code will give the error because the category table hasn't been created yet. Once you have done the first migration, the code will run without error, but as you have noticed, the URLs will not update as new categories are added.
The normal approach is to add a slug kwarg to the URL pattern.
path('applications/', include(('applications.urls', 'applications'), namespace="applications")),
path('<slug:slug>', include('post.urls')),
path('draft/<slug:slug>', include('post.urls')),
Then in the view, use get_object_or_404 to make sure that the category exists.
from django.shortcuts import get_object_or_404
def category_view(request, slug):
category = get_object_or_404(Category, slug=slug)
...
One problem with this, is that <slug:slug>/ will match other URLs, e.g. /image/ or /polls/. In the above code, I've avoided that problem by putting <slug:slug> at the bottom of the URL patterns. Another option would be to use something like categories/<slug:slug>/ so that it doesn't match other URLs.