Django

Django is a high-level Python web framework that encourages rapid development and clean, pragmatic design. It follows the MVT (Model-View-Template) architectural pattern.

A Django project comprises one or more Django applications, each serving a unique function.

django-admin

• Display all available django-admin commands:

django-admin --help

• You can display the help page for each django-admin command:

django-admin help <command_name>

• To start a project:

django-admin startproject <project_name>

manage.py

Run server

Make sure to be in the folder where the manage.py file is located, then run the command to start the server:

python manage.py runserver

Migrations

Each migration is a Python script that makes a change to the database.

• Show list of migrations:

python manage.py showmigrations

• Create migration files:

python manage.py makemigrations

makemigrations generates Python code that represents the steps needed to get the database to the right state.

• Run migrations:

python manage.py migrate

This will run all the pending migration files.

• See the actual SQL that a migration will run:

python manage.py sqlmigrate <app_name> <migration_id>

• Revert migrations:

python manage.py migrate tasks zero

If reverting to the initial migration, you use zero. Otherwise, specify the prefix of the migration file name corresponding to the migration number:

python manage.py migrate tasks 0021

• Create an empty/custom migration file to do something specific:

python manage.py makemigrations tasks --empty

from django import migrations

def change_archived_tasks(apps, schema_editor):
    Task = apps.get_model("tasks", "Task")
    Task.objects.filter(status="ARCHIVED").update(status="COMPLETED")

class Migration(migrations.Migration):
    dependencies = [
        ("tasks", "0001_initial"),
    ]
    operations = [
        migrations.RunPython(change_archived_tasks)
    ]

• Squashing migrations:

python manage.py squashmigrations app_name start_migration_name end_migration_name

Note: If start_migration_name and end_migration_name are omitted, all the migrations for the app are squashed.

Database Interaction

There are two primary ways to interact with databases in Django:

1. Connect to the database shell:

python manage.py dbshell

The dbshell command provides direct access to the database configured in settings.py.

1. Use object managers via Model.objects:
2. .get(): Returns one object from the database. Raises DoesNotExist if not found.

>>> CustomUser.objects.get(id=1)
<CustomUser: admin>

• .filter(): Returns a QuerySet which can contain zero or multiple results.

>>> CustomUser.objects.filter(is_staff=False)
<QuerySet [<CustomUser: seconduser>, <CustomUser: thirduser>]>

Creating Objects from the Shell

python manage.py shell

>>> from django.contrib.auth.models import User
>>> creator = User.objects.create_user("developer", "email@example.com", "password")

Data Migrations

Data migrations change the data but keep the migration schema unchanged. They are useful for mass updating objects.

Print Out the Raw Query

You can inspect the underlying SQL of a QuerySet using the query attribute:

print(Task.objects.exclude(status='ARCHIVED').query)