id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
3779	Problem with ManyToManyFields and syncdb	Ben Slavin <benjamin.slavin@…>	Adrian Holovaty	"The creation of !ManyToMany tables in the database through syncdb is prone to failure, and depends on the ordering of {{{INSTALLED_APPS}}} in {{{settings.py}}}.

{{{manage.py syncdb}}} uses a deferral system ({{{seen_models}}} and {{{pending_references}}}) to ensure proper setup of model tables in the database.  This allows the tables to be created without references, and constraints to be added after the related tables exist.

Currently, the creation of {{{ManyToMany}}} tables (in {{{django.core.management._get_many_to_many_sql_for_model}}}) does not use a deferral mechanism, and will fail to create the tables properly. If the models are loaded in the right order, this isn't an issue; however, if the they are loaded in reverse order and the database is strict about enforcing references (PostgreSQL), this is problematic.

'''Note:''' This doesn't impact MySQL (MyISAM) or sqlite because of the way they handle references -- they don't.

Here's an example:

{{{bar/models.py}}}
{{{
#!python
from django.db import models
from myproj.foo.models import Foo
class Bar(models.Model):
    foos = models.ManyToManyField(Foo)
}}}

{{{foo/models.py}}}
{{{
#!python
from django.db import models
class Foo(models.Model):
    a = models.IntegerField()
}}}

{{{settings.py}}} excerpt ('''working''' version)
{{{
#!python
INSTALLED_APPS = ('m2m.foo','m2m.bar')
}}}

{{{settings.py}}} excerpt ('''non-working''' version)
{{{
#!python
INSTALLED_APPS = ('m2m.bar','m2m.foo')
}}}

{{{manage.py syncdb}}} output for non-working version
{{{
Creating table bar_bar
Traceback (most recent call last):
  File ""./manage.py"", line 11, in ?
    execute_manager(settings)
  File ""/usr/lib/python2.4/site-packages/django/core/management.py"", line 1666, in execute_manager
    execute_from_command_line(action_mapping, argv)
  File ""/usr/lib/python2.4/site-packages/django/core/management.py"", line 1565, in execute_from_command_line
    action_mapping[action](int(options.verbosity), options.interactive)
  File ""/usr/lib/python2.4/site-packages/django/core/management.py"", line 543, in syncdb
    cursor.execute(statement)
  File ""/usr/lib/python2.4/site-packages/django/db/backends/util.py"", line 12, in execute
    return self.cursor.execute(sql, params)
  File ""/usr/lib/python2.4/site-packages/django/db/backends/postgresql/base.py"", line 43, in execute
    return self.cursor.execute(sql, [smart_basestring(p, self.charset) for p in params])
psycopg.ProgrammingError: ERROR:  relation ""foo_foo"" does not exist

CREATE TABLE ""bar_bar_foos"" (
    ""id"" serial NOT NULL PRIMARY KEY,
    ""bar_id"" integer NOT NULL REFERENCES ""bar_bar"" (""id"") DEFERRABLE INITIALLY DEFERRED,
    ""foo_id"" integer NOT NULL REFERENCES ""foo_foo"" (""id"") DEFERRABLE INITIALLY DEFERRED,
    UNIQUE (""bar_id"", ""foo_id"")
);
}}}

The problem is that ""foo_id"" should be ""{{{integer NOT NULL}}}"" and an {{{ALTER TABLE}}} statement should be issued for {{{bar_bar_foos}}} after the {{{foo_foo}}} table is created, rather than trying to create the reference initially."		closed	Core (Other)	dev		fixed			Accepted	0	0	0	0	0	0