Models

• 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.
• Generally, each model maps to a single database table.

The basics:

• Each model is a Python class that subclasses django.db.models.Model.
• Each attribute of the model represents a database field.
• With all of this, Django gives you an automatically-generated database-access API.

Basic example

This example model defines a Person, which has a first_name and last_name:

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

• first_name and last_name are fields of the model.
• Each field is specified as a class attribute, and each attribute maps to a database column.
• The above Person model would create a database table like this:

CREATE TABLE myapp_person (
    "id" serial NOT NULL PRIMARY KEY,
    "first_name" varchar(30) NOT NULL,
    "last_name" varchar(30) NOT NULL
);

Using models

• Once you have defined your models, you need to tell Django you’re going to use those models.
• Do this by editing your settings file and changing the INSTALLED_APPS setting to add the name of the module that contains your models.py.
• For example, if the models for your application live in the module myapp.models (the package structure that is created for an application by the manage.py startapp script), INSTALLED_APPS should read, in part:

INSTALLED_APPS = [
    #...
    'myapp',
    #...
]

• When you add new apps to INSTALLED_APPS, be sure to run manage.py migrate, optionally making migrations for them first with manage.py makemigrations.

Fields

• The most important part of a model – and the only required part of a model – is the list of database fields it defines.
• Fields are specified by class attributes.
• Be careful not to choose field names that conflict with the models API like clean, save, or delete.
• Example:

from django.db import models

class Musician(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    instrument = models.CharField(max_length=100)

class Album(models.Model):
    artist = models.ForeignKey(Musician, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    release_date = models.DateField()
    num_stars = models.IntegerField()