Foreign key constraint fails even when data types are the same - mysql

Question

I'm building a table with a two part foreign key that references a two part key in another table. It isn't working, and I can't see why because the data types line up.
Can't create table 'tsugi.video_comments' (errno: 150)
Here's the code I'm trying to run:
CREATE TABLE `video_comments` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`video_id` varchar(11) NOT NULL DEFAULT '',
`link_id` int(11) NOT NULL,
`videoTime` int(11) NOT NULL,
`comment` text NOT NULL,
`parent` int(11) unsigned DEFAULT NULL,
`private` tinyint(1) DEFAULT NULL,
`replies` int(11) unsigned DEFAULT '0',
`reports` int(11) DEFAULT '0',
`user_id` int(11) NOT NULL,
`displayname` varchar(2048) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `video_ibfk_1` (`link_id`),
KEY `video_ibfk_2` (`user_id`),
CONSTRAINT `video_comments_ibfk_1` FOREIGN KEY (`parent`) REFERENCES `video_comments` (`id`) ON DELETE CASCADE,
CONSTRAINT `video_ibfk_1` FOREIGN KEY (`link_id`) REFERENCES `lti_link` (`link_id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `video_ibfk_2` FOREIGN KEY (`user_id`) REFERENCES `lti_user` (`user_id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `video_key` FOREIGN KEY (`link_id`, `video_id`) REFERENCES `video_ids` (`link_id`, `video_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=285 DEFAULT CHARSET=utf8;
The command runs successfully if I delete the last constraint, so that's where the problem is, not in the other constraints.
Here's the table it's referencing:
CREATE TABLE `video_ids` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`video_id` varchar(11) NOT NULL DEFAULT '',
`link_id` int(11) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `video_key` (`video_id`,`id`),
KEY `link_id` (`link_id`),
KEY `id` (`id`,`video_id`),
CONSTRAINT `video_ids_ibfk_1` FOREIGN KEY (`link_id`) REFERENCES `t_lti_link` (`link_id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
To make sure that the video_id and link_id fields are exactly the same, I copied them directly from the existing table's code to the code for creating the new table. I expected that to solve it, but it did not. Here's the error log:
------------------------
LATEST FOREIGN KEY ERROR
------------------------
141114 22:04:09 Error in foreign key constraint of table tsugi/video_comments:
FOREIGN KEY (`link_id`, `video_id`) REFERENCES `video_ids` (`link_id`, `video_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=285 DEFAULT CHARSET=utf8:
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.
Note that the internal storage type of ENUM and SET changed in
tables created with >= InnoDB-4.1.12, and such columns in old tables
cannot be referenced by such columns in new tables.
See http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
for correct foreign key definition.

Answer 1

Foreign key references need to be to unique or primary keys. So, you can fix this just by having a unique declaration in video_ids:
CREATE TABLE `video_ids` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`video_id` varchar(11) NOT NULL DEFAULT '',
`link_id` int(11) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `video_key` (`video_id`,`id`),
KEY `link_id` (`link_id`),
KEY `id` (`id`,`video_id`),
UNIQUE (link_id, video_id),
CONSTRAINT `video_ids_ibfk_1` FOREIGN KEY (`link_id`) REFERENCES `t_lti_link` (`link_id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
Here is a SQL Fiddle with an example.