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mysql> show create table module_permissions;                                                                     +--------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table              | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                               |
+--------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| module_permissions | CREATE TABLE `module_permissions` (
  `module` int(3) unsigned NOT NULL default '0',
  `group` int(3) unsigned NOT NULL default '0',
  `permission` varchar(30) NOT NULL default '',
  `value` tinyint(1) NOT NULL default '0',
  UNIQUE KEY `module_group_permission` (`module`,`group`,`permission`),
  KEY `group` (`group`),
  KEY `module` (`module`),
  KEY `module_group` (`module`,`group`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+--------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table module_acls;                                                                            +-------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table       | Create Table                                                                                                                                                                                                                                                            |
+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| module_acls | CREATE TABLE `module_acls` (
  `module` int(3) unsigned NOT NULL default '0',
  `group` int(3) unsigned NOT NULL default '0',
  UNIQUE KEY `module_group` (`module`,`group`),
  KEY `group` (`group`),
  KEY `module` (`module`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> alter table module_permissions add foreign key module_acl_ref (module_group) references module_acls (module_group);
ERROR 1072 (42000): Key column 'module_group' doesn't exist in table
mysql>