Why is this foreign key constraint not working? - mysql

Question

I'm creating a new table who will have relationships with an existing table. The new table has a column user_id which references a userid column on another table (the ID of a user in the users table).
I have the following table:
CREATE TABLE `ads` (
`id` INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
-- Other columns...
)
DEFAULT CHARACTER SET utf8
COLLATE utf8_unicode_ci
ENGINE = InnoDB;
Adding a referencing column
ALTER TABLE `ads` ADD `user_id` INT(11) NOT NULL
Which references the following column
CREATE TABLE users
(
userid INT(11) PRIMARY KEY NOT NULL AUTO_INCREMENT,
-- Other irrelevant fields...
);
And finally my key definition:
ALTER TABLE `ads` ADD CONSTRAINT ads_user_id_foreign FOREIGN KEY (`user_id`) REFERENCES `users` (`userid`)
But for some reason I still get an error:
SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table `ads` add constraint ads_user_id_foreign foreign key (`user_id`) references `users` (`userid`))
Note: I don't believe it's of much relevance, but I'm using Laravel's query builder to create the new table. I'm getting these queries from PHPStorm's Copy DDL feature to show you the equivalent "vanilla MySQL query".

Answer 1

Okay, so I figured it out. In my case, I also needed to change the engine, charset, and collation of the users table. IT was set to MyISAM, latin1, and latin1_swedish_ci, respectively.
I did say I was using Laravel, so here are the statements I used to fix that:
DB::statement('ALTER TABLE users CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci');
DB::statement('ALTER TABLE users ENGINE=InnoDB;');
I'll try to leave a generic answer here for those who find this post in the future.
Things to check when foreign keys aren't working:
1. Make sure that the columns are setup identically
I use an IDE (PHPStorm) that allows me to get some DDL from existing tables. I paste that somewhere and find the columns in question to see if they're defined correctly.
2. Check table status for inconsistencies
You can check all tables' statuses with SHOW TABLE STATUS. You'll get a result that looks like this (obviously this depends on your IDE):
Make sure that engine and collation match in both tables
• If you notice that your engine is wrong (it should be InnoDB), you can fix it with
ALTER TABLE <table_name> ENGINE=InnoDB;
• If you notice that your collation is wrong, you can fix it with
alter table <table_name> convert to character set <CHARSET> collate <COLLATION>;
(where a charset might be utf8 and a collation might be utf8_unicode_ci)