IntegrityError _id may not be NULL - django

Question

Just starting with Django and have began creating an application for test purposes. However, when I specify rollercoasters to ManyToMany and add an instance of a Park via the admin UI I get an:
IntreityError, rollercoasters_id may not be NULL
I have just completed a book on this and the code is more or less exact as the books (the code works in the book).
What am I doing wrong?
The application works when I have ForeignKey instead of ManyToMany
from django.db import models
class Company(models.Model):
name=models.CharField(max_length=30)
location=models.CharField(max_length=30)
website=models.URLField()
def __unicode__(self):
return self.name
class Constructor(models.Model):
name=models.CharField(max_length=30)
location=models.CharField(max_length=30)
contactnumber=models.CharField(max_length=30)
website=models.URLField()
def __unicode__(self):
return self.name
class RollerCoasters(models.Model):
name=models.CharField(max_length=30)
dateopenned=models.DateField
type=models.CharField(max_length=30)
builtby=models.ForeignKey(Constructor)
dateopenned=models.DateField()
def __unicode__(self):
return self.name
class Parks (models.Model):
name=models.CharField(max_length=30)
parent=models.ForeignKey(Company)
location=models.CharField(max_length=1000)
rollercoasters=models.ManyToManyField(RollerCoasters)
dateopenned=models.DateField()
def __unicode__(self):
return self.name
Error:
Environment:
Request Method: POST
Request URL: http://127.0.0.1:8000/admin/Parks/parks/add/
Django Version: 1.4.2
Python Version: 2.7.3
Installed Applications:
('django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.sites',
'django.contrib.messages',
'django.contrib.staticfiles',
'Parks',
'django.contrib.admin',
'django.contrib.admindocs')
Installed Middleware:
('django.middleware.common.CommonMiddleware',
'django.contrib.sessions.middleware.SessionMiddleware',
'django.middleware.csrf.CsrfViewMiddleware',
'django.contrib.auth.middleware.AuthenticationMiddleware',
'django.contrib.messages.middleware.MessageMiddleware')
Traceback:
File "C:\Python27\lib\site-packages\django\core\handlers\base.py" in get_response
111. response = callback(request, *callback_args, **callback_kwargs)
File "C:\Python27\lib\site-packages\django\contrib\admin\options.py" in wrapper
366. return self.admin_site.admin_view(view)(*args, **kwargs)
File "C:\Python27\lib\site-packages\django\utils\decorators.py" in _wrapped_view
91. response = view_func(request, *args, **kwargs)
File "C:\Python27\lib\site-packages\django\views\decorators\cache.py" in _wrapped_view_func
89. response = view_func(request, *args, **kwargs)
File "C:\Python27\lib\site-packages\django\contrib\admin\sites.py" in inner
196. return view(request, *args, **kwargs)
File "C:\Python27\lib\site-packages\django\utils\decorators.py" in _wrapper
25. return bound_func(*args, **kwargs)
File "C:\Python27\lib\site-packages\django\utils\decorators.py" in _wrapped_view
91. response = view_func(request, *args, **kwargs)
File "C:\Python27\lib\site-packages\django\utils\decorators.py" in bound_func
21. return func(self, *args2, **kwargs2)
File "C:\Python27\lib\site-packages\django\db\transaction.py" in inner
209. return func(*args, **kwargs)
File "C:\Python27\lib\site-packages\django\contrib\admin\options.py" in add_view
955. self.save_model(request, new_object, form, False)
File "C:\Python27\lib\site-packages\django\contrib\admin\options.py" in save_model
709. obj.save()
File "C:\Python27\lib\site-packages\django\db\models\base.py" in save
463. self.save_base(using=using, force_insert=force_insert, force_update=force_update)
File "C:\Python27\lib\site-packages\django\db\models\base.py" in save_base
551. result = manager._insert([self], fields=fields, return_id=update_pk, using=using, raw=raw)
File "C:\Python27\lib\site-packages\django\db\models\manager.py" in _insert
203. return insert_query(self.model, objs, fields, **kwargs)
File "C:\Python27\lib\site-packages\django\db\models\query.py" in insert_query
1593. return query.get_compiler(using=using).execute_sql(return_id)
File "C:\Python27\lib\site-packages\django\db\models\sql\compiler.py" in execute_sql
910. cursor.execute(sql, params)
File "C:\Python27\lib\site-packages\django\db\backends\util.py" in execute
40. return self.cursor.execute(sql, params)
File "C:\Python27\lib\site-packages\django\db\backends\sqlite3\base.py" in execute
344. return Database.Cursor.execute(self, query, params)
Exception Type: IntegrityError at /admin/Parks/parks/add/
Exception Value: Parks_parks.rollercoasters_id may not be NULL

Answer 1

You can't just swap between a foreign key and many to many field. If you have a foreign key, Django creates a database column e.g. rollercoaster_id. If you use a many to many, Django creates an intermediary join table.
To switch from a foreign key to a many to many field, you must do one of the following:
drop the tables and then run syncdb so that Django recreates them. You will lose any data.
manually update the database schema yourself, using the output of the manage.py sql command as a guide
use the schema migration tool South.