Some findings in Django

How to use settings and introspects Django database models.

Photo by Hal Gatewood on Unsplash

Recently, I was asked as an exercise for a job interview to create a Django package that introspects a project at its startup time and generates a file with all the database models encountered. The final result can be found here. I will share what I learned from this project.

Note: The following content will only be useful for you if you already have the basics in Django. If it is not the case, I encourage you to read their excellent tutorial.

How to effectively access project settings.

A naive way to think about how to retrieve the project settings is to access directly the settings module found in the project. But this is a bad idea because not all settings used by Django are found in that module. Some of them are defined in django.conf.global_settings. The correct way to access Django settings is like the following:

from django.conf import settings

if settings.DEBUG:
    # Do something

Don’t try this:

from django.conf.settings import DEBUG

The previous code will not work because settings here is an object and not a module.

Also, don’t try to alternate settings at runtime, you may experience unexpected behaviors.

from django.conf import settings

settings.DEBUG = True   # Don't do this!

The only way to assign to settings is in the settings file.

Instropect a Django Model

I was asked to inspect models and retrieve the field names and types. I found that each Django model has a_meta attribute (which is an instance of the django.db.models.Optionclass) I can use for my goal. It has a method get_fields that I can use like the following:

# model here is a django db.Model class like User, Comment,...
for field in model._meta.get_fields():
    yield field.name, field.__class__.__name__

Get all concrete models in a models module

Another thing to take into account is that I don’t want to include abstract or proxy models in the resulting file because these models don’t have a representation in a database. Again all this information can be found in the _metaattribute. So the logic to get all concrete models in the project is as follows:

# we assume we have a function to iterate all models

for model in get_models():
    if not model._meta.abstract and not model._meta.proxy:
        for field in model._meta.get_fields():
            yield field.name, field.__class__.__name__

Do something when a Django project starts running

Another requirement was to scan the project for models when it starts running. Normally all Django applications have an apps module with a class that inherits from django.apps.AppConfig. This class has an empty ready function that is intended to be overridden to execute code at project startup.

from django.apps import AppConfig

class ScanormConfig(AppConfig):
    name = 'scanorm'

    def ready(self):
        # All you write here will be executed when the project starts running
        create_model_schema()

This is all for this article. Hope you enjoyed it. Take care of yourself and see you next time! 😁

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