Much better Django integration

[ewjoachim]

Closes #363

Successful PR Checklist:

• Tests
  • (not applicable?)
• Documentation
  • (not applicable?)

Tasks from #363

• pip install procrastinate[django]

  • Already good

• Add "procrastinate.contrib.django" to INSTALLED_APPS

  • Already good

• Tweak one or two PROCRASTINATE_* variables inside Django's settings.py file

  • Done

• Create tasks inside myapp/tasks.py, decorating them with procrastinate.task

  • I've chosen to use from procrastinate.contrib.django import app, @app.task

• Run the worker with python manage.py procrastinate-worker (set DJANGO_SETTINGS_MODULE env var when running the worker)

  • I've chosen to use python manage.py procrastinate worker instead. Note that manage.py already
    sets DJANGO_SETTINGS_MODULE.

• Monitor tasks inside Django Admin (running, finished, failed, waiting etc.)

  • For now it's the bare minimum. We can add proper filters etc in a subsequent PR.

• Fix existing tests

• Write new tests (especially, Django tests, and "Django ORM in tasks" tests including async ORM)

Breaking change announcement draft

This release refactors the Django integration completely. Head to the Django section in the doc to discover how it works.
Here's a summary of the changes:

• You don't need to define an app anymore. As long as procrastinate is in your INSTALLED_APPS, you can use procrastinate.contrib.django.app. The app will be opened for you at the start of the process (during Django's setup) and will be properly closed when the process stops.
• Tasks defined under the tasks submodule of any app in INSTALLED_APPS are loaded by default. This is configurable.
• Some settings can be defined as PROCRASTINATE_* in your Django settings. They include specifying where your tasks are (among other things)
• You can run the CLI with ./manage.py procrastinate (if you use that, you won't have to configure an App either)
• Procrastinate now exposes 2 models and the corresponding ModelAdmins in your Django Admin. Those models are read-only. The Admin panels are very very basic for now.
• Procrastinate is tested with Django tasks that call the classic django ORM and the new async ORM methods.

PR label(s):

• PR type: breaking 💥 Contains breaking changes
• PR type: feature ⭐️ Contains new features
• PR type: bugfix 🕵️ Contains bug fix
• PR type: miscellaneous 👾 Contains misc changes
• PR type: dependencies 🤖 Contains only dependencies updates
• PR type: documentation 📚 Contains documentation updates

[ewjoachim]

@turicas you will probably be interested by this PR. If you want more info, feel free to ask here or on the shiny new discord server (link at the top of the project README)

I'll need to fix the test, but otherwise, it's starting to look great.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
procrastinate
cli.py
connector.py
psycopg_connector.py
sync_psycopg_connector.py
utils.py
procrastinate/contrib/aiopg
aiopg_connector.py
procrastinate/contrib/django
__init__.py
apps.py					26
django_connector.py					19-20
exceptions.py
models.py
router.py					12-13
utils.py
procrastinate/contrib/django/management/commands
procrastinate.py					17-18, 34-36, 41-48
Project Total

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