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.