Django Models

LINKS

• Using Models

• Django Admin

• Beginner’s Guide to Django - Part 1
• Video: Beginner’s Guide to Django - Part 2

Using Models

Model Definition

Models are usually defined in an application’s models.py file. They are implemented as subclasses of django.db.models.Model, and can include fields, methods and metadata. The code fragment below shows a “typical” model, named MyModelName:

from django.db import models

class MyModelName(models.Model):
    """A typical class defining a model, derived from the Model class."""

    # Fields
    my_field_name = models.CharField(max_length=20, help_text='Enter field documentation')
    ...

    # Metadata
    class Meta:
        ordering = ['-my_field_name']

    # Methods
    def get_absolute_url(self):
        """Returns the url to access a particular instance of MyModelName."""
        return reverse('model-detail-view', args=[str(self.id)])

    def __str__(self):
        """String for representing the MyModelName object (in Admin site etc.)."""
        return self.my_field_name

Common field arguments

The following common arguments can be used when declaring many/most of the different field types:

• help_text: Provides a text label for HTML forms (e.g. in the admin site), as described above.
• verbose_name: A human-readable name for the field used in field labels. If not specified, Django will infer the default verbose name from the field name.
• default: The default value for the field. This can be a value or a callable object, in which case the object will be called every time a new record is created.
• null: If True, Django will store blank values as NULL in the database for fields where this is appropriate (a CharField will instead store an empty string). The default is False.
• blank: If True, the field is allowed to be blank in your forms. The default is False, which means that Django’s form validation will force you to enter a value. This is often used with null=True , because if you’re going to allow blank values, you also want the database to be able to represent them appropriately.
• choices: A group of choices for this field. If this is provided, the default corresponding form widget will be a select box with these choices instead of the standard text field.
• primary_key: If True, sets the current field as the primary key for the model (A primary key is a special database column designated to uniquely identify all the different table records). If no field is specified as the primary key then Django will automatically add a field for this purpose.

Django Admin

Registering models

First, open admin.py in the catalog application (/locallibrary/catalog/admin.py). It currently looks like this — note that it already imports django.contrib.admin:

from django.contrib import admin

# Register your models here.

Register the models by copying the following text into the bottom of the file. This code imports the models and then calls admin.site.register to register each of them.

from .models import Author, Genre, Book, BookInstance

admin.site.register(Book)
admin.site.register(Author)
admin.site.register(Genre)
admin.site.register(BookInstance)

This is the simplest way of registering a model, or models, with the site. The admin site is highly customisable, and we’ll talk more about the other ways of registering your models further down.

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