通过数据库向Django模型添加字段的示例

from django.db import models

class Publisher(models.Model):
  name = models.CharField(max_length=30)
  address = models.CharField(max_length=50)
  city = models.CharField(max_length=60)
  state_province = models.CharField(max_length=30)
  country = models.CharField(max_length=50)
  website = models.URLField()

  def __unicode__(self):
    return self.name

class Author(models.Model):
  first_name = models.CharField(max_length=30)
  last_name = models.CharField(max_length=40)
  email = models.EmailField()

  def __unicode__(self):
    return u'%s %s' % (self.first_name, self.last_name)

class Book(models.Model):
  title = models.CharField(max_length=100)
  authors = models.ManyToManyField(Author)
  publisher = models.ForeignKey(Publisher)
  publication_date = models.DateField()

  def __unicode__(self):
    return self.title

class Book(models.Model):
  title = models.CharField(max_length=100)
  authors = models.ManyToManyField(Author)
  publisher = models.ForeignKey(Publisher)
  publication_date = models.DateField()
  **num_pages = models.IntegerField(blank=True, null=True)**

  def __unicode__(self):
    return self.title

CREATE TABLE "books_book" (
  "id" serial NOT NULL PRIMARY KEY,
  "title" varchar(100) NOT NULL,
  "publisher_id" integer NOT NULL REFERENCES "books_publisher" ("id"),
  "publication_date" date NOT NULL,
  "num_pages" integer NULL
);

"num_pages" integer NULL

ALTER TABLE books_book ADD COLUMN num_pages integer;

BEGIN;
ALTER TABLE books_book ADD COLUMN num_pages integer;
UPDATE books_book SET num_pages=0;
ALTER TABLE books_book ALTER COLUMN num_pages SET NOT NULL;
COMMIT;

>>> from mysite.books.models import Book
>>> Book.objects.all()[:5]