Unable to alter table with foreign key

Question

My two empty tables:

CREATE TABLE person (
    person_id SMALLINT UNSIGNED,
    fname VARCHAR(20),
    lname VARCHAR(20),
    gender ENUM('m', 'f'),
    birth_date DATE,
    street VARCHAR(30),
    city VARCHAR(20),
    state VARCHAR(20),
    country VARCHAR(20),
    postal_code VARCHAR(20),
    CONSTRAINT pk_person PRIMARY KEY (person_id)
);

create TABLE favorite_food (
    person_id SMALLINT UNSIGNED,
    food VARCHAR(20),
    CONSTRAINT pk_favorite_food PRIMARY KEY (person_id, food),
    CONSTRAINT fk_fav_food_person_id FOREIGN KEY (person_id) REFERENCES person (person_id)
);

Needed modification:

ALTER TABLE person MODIFY person_id SMALLINT UNSIGNED AUTO_INCREMENT;

Result:

ERROR 1833 (HY000): Cannot change column 'person_id': used in a foreign key constraint 'fk_fav_food_person_id' of table 'tom.favorite_food'

Why is this and is there a way around this without dropping the tables and redefining them?

Answer 1

This is probably because there already is data in person.person_id in any rows (NOT NULL). You can circumvent this by disabling foreign key checks

Answer 2

I believe the best way to handle this is to drop the foreign key reference from favorite_food, alter the column in person and then recreate the foreign key reference. That will properly recreate the index on which the key depends.