server/settings.py
By default, the configuration uses SQLite. SQLite is included in Python, so no need to install anything else to support your database.
To use another database, we need to install the appropriate database bindings and change the "ENGINE" and "NAME" keys in the DATABASES 'default' item to match your database connection settings:
ENGINE – 'django.db.backends.sqlite3', 'django.db.backends.postgresql', 'django.db.backends.mysql', etc.
NAME – The name of your database. The default value, BASE_DIR / 'db.sqlite3', will store the file in your project directory.
Note, the INSTALLED_APPS setting at the top of the file. That holds the names of all Django applications that are activated in this Django-instance.
django.contrib.admin – The admin site. You’ll use it shortly.
django.contrib.auth – An authentication system.
django.contrib.contenttypes – A framework for content types.
django.contrib.sessions – A session framework.
django.contrib.messages – A messaging framework.
django.contrib.staticfiles – A framework for managing static files.
Some of these applications make use of at least one database table, though, so we need to create the tables in the database before we can use them. To do that, run the following command:
$ python manage.py migrate
The migrate command looks at the INSTALLED_APPS setting and creates any necessary database tables according to the database settings in your mysite/settings.py file and the database migrations shipped with the app.
Models – essentially, your database layout, with additional metadata.
A model is the single, definitive source of information about your data. It contains the essential fields and behaviors of the data you’re storing. Django follows the DRY Principle. The goal is to define your data model in one place and automatically derive things from it.
This includes the migrations - unlike in Ruby On Rails, for example, migrations are entirely derived from your models file, and are essentially a history that Django can roll through to update your database schema to match your current models.
In our poll app, we’ll create two models: Question and Choice. A Question has a question and a publication date. A Choice has two fields: the text of the choice and a vote tally. Each Choice is associated with a Question.
These concepts are represented by Python classes.
personalization/models.py
from django.db import models
class Question(models.Model):
question_text = models.CharField(max_length=200)
pub_date = models.DateTimeField('date published')
class Choice(models.Model):
question = models.ForeignKey(Question, on_delete=models.CASCADE)
choice_text = models.CharField(max_length=200)
votes = models.IntegerField(default=0)
- Each model is represented by a class that subclasses django.db.models.Model. Each model has a number of class variables, each of which represents a database field in the model.
- Each field is represented by an instance of a Field class – e.g., CharField for character fields and DateTimeField for datetimes. This tells Django what type of data each field holds.
- Some Field classes have required arguments. CharField, for example, requires that you give it a max_length. That’s used not only in the database schema, but in validation.
- A Field can also have various optional arguments; in this case, we’ve set the default value of votes to 0.
Finally, note a relationship is defined, using ForeignKey. That tells Django each Choice is related to a single Question. Django supports all the common database relationships: many-to-one, many-to-many, and one-to-one.
That small bit of model code gives Django a lot of information. With it, Django is able to:
- Create a database schema (CREATE TABLE statements) for this app.
- Create a Python database-access API for accessing Question and Choice objects.
But first we need to tell our project that the polls app is installed.
Django apps are “pluggable”: You can use an app in multiple projects, and you can distribute apps, because they don’t have to be tied to a given Django installation.
server/settings.py
INSTALLED_APPS = [
'personalization.apps.PersonalizationConfig',
'django.contrib.admin',
...
]
Now Django knows to include the polls app. Let’s run another command:
$ python manage.py makemigrations polls
By running makemigrations, you’re telling Django that you’ve made some changes to your models (in this case, you’ve made new ones) and that you’d like the changes to be stored as a migration.
Migrations are how Django stores changes to your models (and thus your database schema) - they’re files on disk. You can read the migration for your new model if you like; it’s the file polls/migrations/0001_initial.py. Don’t worry, you’re not expected to read them every time Django makes one, but they’re designed to be human-editable in case you want to manually tweak how Django changes things.
The migrate command takes all the migrations that haven’t been applied (Django tracks which ones are applied using a special table in your database called django_migrations) and runs them against your database - essentially, synchronizing the changes you made to your models with the schema in the database.
Migrations are very powerful and let you change your models over time, as you develop your project, without the need to delete your database or tables and make new ones - it specializes in upgrading your database live, without losing data.
- Change your models (in models.py).
- Run pythonmanage.pymakemigrations to create migrations for those changes
- Run pythonmanage.pymigrate to apply those changes to the database.
The reason that there are separate commands to make and apply migrations is because you’ll commit migrations to your version control system and ship them with your app; they not only make your development easier, they’re also usable by other developers and in production.
