MySQL - convert MyISAM into InnoDB getting error 1075 - mysql

Question

I am trying to convert a table from MyISAM into InnoDB, this is the definition and I am getting error #1075 - Incorrect table definition; there can be only one auto column and it must be defined as a key
The table has an AutoIncrement value and the field is indexed and it works with MyISAM. I am new to InnoDB so it might be a dumb question
CREATE TABLE `cart_item` (
`cart_id` int(10) unsigned NOT NULL DEFAULT '0',
`id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
`design_number` int(10) unsigned NOT NULL,
`logo_position_id` smallint(5) unsigned NOT NULL,
`subst_style_id` varchar(10) DEFAULT NULL,
`style_id` varchar(10) NOT NULL DEFAULT '',
`subst_color_id` smallint(5) unsigned DEFAULT NULL,
`color_id` smallint(5) unsigned NOT NULL,
`size_id` smallint(5) unsigned NOT NULL,
`qty` mediumint(8) unsigned NOT NULL,
`active` enum('y','n') NOT NULL DEFAULT 'y',
`date_last_modified` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`last_modified_by_id` mediumint(5) unsigned NOT NULL,
`date_last_locked` datetime DEFAULT NULL,
`last_locked_by_id` smallint(5) unsigned NOT NULL,
`date_added` datetime NOT NULL,
`subsite_logo_group_id` int(11) NOT NULL,
`bundle` varchar(32) NOT NULL,
`color_stop_1` varchar(4) DEFAULT NULL,
PRIMARY KEY (`cart_id`,`id`),
KEY `color_id` (`color_id`),
KEY `style_id` (`style_id`),
KEY `size_id` (`size_id`),
KEY `design_number` (`design_number`),
KEY `subsite_logo_group_id` (`subsite_logo_group_id`),
KEY `date_added` (`date_added`),
KEY `bundle` (`bundle`)
) ENGINE=InnoDB

Answer 1

What you were doing on the MyISAM table, cannot be done with InnoDB. See my answer on a (similar) problem: creating primary key based on date
MySQL docs, in the Using AUTO_INCREMENT section, explain it:
For MyISAM tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.
You may get similar behaviour in InnoDB but not with AUTO_INCREMENT. You'll have to use either some fancy trigger or a stored procedure for your Inserts that will take care of the (per cart_id) auto-increment.

Answer 2

You have a composite PRIMARY KEY defined on (cart_id, id), but the AUTO_INCREMENT requires an index on id alone. You can add a KEY for it (not a primary key, but just a plain index):
KEY `idx_id` (`id`)
I question the use of the composite PK on (cart_id, id) though, since id is alone a unique value by definition. Perhaps you should make id the PK, and create a separate index across the combination.
PRIMARY KEY (`id`),
KEY (`cart_id`, `id`)
It doesn't even need to be specified as UNIQUE because the AUTO_INCREMENT can't be repeated anyway. There is no way to violate uniqueness on the combination (cart_id, id).

Answer 3

AUTO_INCREMENT columns should be define as key, as what the error implies.
`id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
and set UNIQUE on the two column instead of primary key
UNIQUE (`cart_id`,`id`),
SQLFiddle Demo