Writing your first Django app, part 2

Database setup

Now, open up mysite/settings.py.

By default, the configuration uses SQLite. SQLite is included in Python, so you won’t need to install anything else to support your database while starting your first real project .

If you wish to use another database, install the appropriate database and change the following keys in the DATABASES 'default' item to match your database connection settings:

• ENGINE – Either 'django.db.backends.sqlite3', 'django.db.backends.postgresql', 'django.db.backends.mysql', or 'django.db.backends.oracle'. Other backends are also available.
• NAME – The name of your database. If you’re using SQLite, the database will be a file on your computer; in that case, NAME should be the full absolute path, including filename, of that file. The default value, os.path.join(BASE_DIR, 'db.sqlite3'), will store the file in your project directory.

If you are not using SQLite as your database, additional settings such as USER, PASSWORD, and HOST must be added.


This example is for PostgreSQL:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql',
        'NAME': 'mydatabase',
        'USER': 'mydatabaseuser',
        'PASSWORD': 'mypassword',
        'HOST': '127.0.0.1',
        'PORT': '5432',
    }
}

ENGINE¶

The database backend to use. The built-in database backends are:

• 'django.db.backends.postgresql'
• 'django.db.backends.mysql'
• 'django.db.backends.sqlite3'
• 'django.db.backends.oracle'

While you’re editing mysite/settings.py, set TIME_ZONE to your time zone.

Also, 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 .

ute them for use by others in their projects.
By default, INSTALLED_APPS contains the following apps, all of which come with Django:

• 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.

These applications are included by default as a convenience for the common case.

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 .


Note :The migrate command will only run migrations for apps in INSTALLED_APPS.

Creating models :

Now we’ll define your models – essentially, your database layout, with additional metadata.

A model contains the essential fields and behaviors of the data you’re storing .

In our simple 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 simple Python classes. Edit the polls/models.py file so it looks like this:

polls/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)

Activating models

 First we need to tell our project that the polls app is installed.

To include the app in our project, we need to add a reference to its configuration class in the INSTALLED_APPS setting. 

The PollsConfig class is in the polls/apps.py file, so its dotted path is 'polls.apps.PollsConfig'. 

Edit the mysite/settings.py file and add that dotted path to the INSTALLED_APPS setting.

INSTALLED_APPS = [
    'polls.apps.PollsConfig',

] 

Now Django knows to include the polls app. Let’s run another command:

$ python manage.py makemigrations polls

You should see something similar to the following:

Migrations for 'polls':
  polls/migrations/0001_initial.py:
    - Create model Choice
    - Create model Question
    - Add field question to choice

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 just files on disk .

Note the following:

Table names are automatically generated by combining the name of the app (polls) and the lowercase name of the model – question and choice. (You can override this behavior.)

Primary keys (IDs) are added automatically. (You can override this, too.)

By convention, Django appends "_id" to the foreign key field name. (Yes, you can override this, as well.)

The foreign key relationship is made explicit by a FOREIGN KEY constraint

The sqlmigrate command doesn’t actually run the migration on your database - it just prints it to the screen so that you can see what SQL Django thinks is required. It’s useful for checking what Django is going to do or if you have database administrators who require SQL scripts for changes.

$ python manage.py sqlmigrate polls 0001

If you’re interested, you can also run 

python manage.py check; 

this checks for any problems in your project without making migrations or touching the database.

Now, run migrate again to create those model tables in your database:

$ python manage.py migrate

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.

remember the three-step guide to making model changes:

• Change your models (in models.py).
• Run python manage.py makemigrations to create migrations for those changes
• Run python manage.py migrate to apply those changes to the database.