Using Django Check Constraints to Limit A Model to a Single Instance
Yet another use case for creating a database constraint with Django’s CheckConstraint class.
(All tags.)
Yet another use case for creating a database constraint with Django’s CheckConstraint class.
Here’s another use case for creating a database constraint with Django’s CheckConstraint class.
I recently optimized a client project’s test suite, and in the process found a test whose runtime had crept up ever since it had been written. The problematic test exercised an import process from a fixed past date until the current day. The test’s runtime therefore grew every day, until it reached over a minute.
For my new Django project, DB Buddy, I’m using CloudFlare as my CDN. It has a bunch of useful features that would otherwise take extra work, such as DDoS protection, HTML minification, and analytics.
A client project recently was suffering from an N+1 queries problem in a complicated Django admin page. Many measures had already been taken to prevent N+1 queries, such as use of django-auto-prefetch and some manually tuned select_related() / prefetch_related() calls. The remaining N+1 in question was a bit resistant to those methods because it came through several layers of admin code and many-to-many fields, making it harder than normal to find the place to modify the QuerySet construction.
If you start the development server on a Django project, it looks something like this:
In recent years browsers have gained many powers to report back problems they encounter on your site, such as:
Today I learned about the better-exceptions package. It makes exception output better, providing more context and colourization on the terminal.
In all current releases of the popular WSGI server gunicorn, the Server header reports the complete version of gunicorn. I spotted this on my new project DB Buddy. For example, with httpie to check the response headers:
It can be tricky to ensure all the environments that your project runs on use the same versions of Python, PostgreSQL, and other external dependencies. Often development, CI, and cloud environments have different configuration systems, making them hard to keep in sync. And coordinating between all your team members to upgrade their local environments can be complicated, as upgrade emails or instant messages get forgotten if they are away on holiday, working on other projects, etc. And using the wrong versions of external dependencies can lead to hard-to-debug errors, wasting time to find such a simple fix.
If you’ve used Django migrations for a while, you may be familiar with this message:
Earlier this week I tweeted about my two part offer on my book Speed Up Your Django Tests for this year’s Cyber Monday.
I previously covered writing a Django application in a single file, for both synchronous and asynchronous use cases. This post covers the angle of creating a REST API using Django in a single file.
Last week, I released a new Django package, django-read-only. It provides a read-only mode for Django’s database layer.
When we write custom management commands, it’s easy to write integration tests for them with call_command(). This allows us to invoke the management command as it runs under manage.py, and retrieve the return code, standard output, and standard error. It’s great, but has some overhead, making our tests slower than necessary. If we have logic separated out of the command’s handle() method, it improves both readability and testability, as we can unit test it separately.
You run your tests with manage.py test. You know what happens inside your tests, since you write them. But how does the test runner work to execute them, and put the dots, Es, and Fs on your screen?
There are a few mini sites out there with “bonus” Django documentation. Here’s a list of the best ones I know of.
The N+1 Queries Problem is a perennial database performance issue. It affects many ORMs and custom SQL code, and Django’s ORM is not immune either.
Last week I covered Django’s database instrumentation, and making a wrapper that’s always installed. Here’s a different use case that I encountered last year on a project.
If you’ve read the Django documentation for Model.Meta.index_together recently, you may have noticed this note:
Since version 2.0, Django has provided a hook for installing database instrumentation wrapper functions. These functions are like middleware for database queries, allowing you to inspect and rewrite SQL before it is sent to the database. There are many use cases, for example:
Django’s test command takes the --parallel flag to activate parallel testing. Unfortunately, this has never worked on Windows, and with Python 3.8 it has stopped working on macOS.
Here’s a conversation I had earlier with the late Ernest Hemingway:
Your Django project may use some packages that add instrumentation, for example:
Since I launched my book Speed Up Your Django Tests I’ve had a number of questions about possible discounts. I want to make my book affordable for all and understand that $49 is a high price tag for some. The Django community is global and I want to share knowledge with Djangonauts everywhere.