$ python manage.py makemigrations
Migrations for 'books':
 0003_auto.py:
  - Alter field author on book

$ python manage.py migrate
Operations to perform:
 Synchronize unmigrated apps: sessions, admin, messages, auth, staticfiles, contenttypes
 Apply all migrations: books
Synchronizing apps without migrations:
 Creating tables...
 Installing custom SQL...
 Installing indexes...
Installed 0 object(s) from 0 fixture(s)
Running migrations:
 Applying books.0003_auto... OK

$ python manage.py makemigrations your_app_label

python manage.py makemigrations --empty yourappname

# -*- coding: utf-8 -*-
from django.db import models, migrations 
class Migration(migrations.Migration): 
  dependencies = [
    ('yourappname', '0001_initial'),
  ] 
  operations = [
  ]

# -*- coding: utf-8 -*-
from django.db import models, migrations
 
def combine_names(apps, schema_editor):
  # We can't import the Person model directly as it may be a newer
  # version than this migration expects. We use the historical version.
  Person = apps.get_model("yourappname", "Person")
  for person in Person.objects.all():
    person.name = "%s %s" % (person.first_name, person.last_name)
    person.save()
 
class Migration(migrations.Migration):
 
  dependencies = [
    ('yourappname', '0001_initial'),
  ]
 
  operations = [
    migrations.RunPython(combine_names),
  ]

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
);

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;
UPDATE books_book SET num_pages = NULL;
COMMIT;

ALTER TABLE <YourTable> ADD <NewColumn> <NewColumnType> NOT NULL DEFAULT <DefaultValue>;

ALTER TABLE books_book DROP COLUMN num_pages;

BEGIN TRANSACTION;
CREATE TEMPORARY TABLE t1_backup(a,b);
INSERT INTO t1_backup SELECT a,b FROM t1;
DROP TABLE t1;
CREATE TABLE t1(a,b);
INSERT INTO t1 SELECT a,b FROM t1_backup;
DROP TABLE t1_backup;
COMMIT;

DROP TABLE books_book_authors;

DROP TABLE books_book;

python manage.py dumpdata myapp > myapp.json

[
 {
  "model": "myapp.person",
  "pk": 1,
  "fields": {
   "first_name": "John",
   "last_name": "Lennon"
  }
 },
 {
  "model": "myapp.person",
  "pk": 2,
  "fields": {
   "first_name": "Paul",
   "last_name": "McCartney"
  }
 }
]

python manage.py loaddata myapp.json

python manage.py dumpdata auth > auth.json