MySQL: update on a table with multiple foreign key dependencies - mysql

Question

Below is the simplified picture of the relationships I have in my DB:
create table attribute (id int auto_increment, primary key (id));
create table state_sample (id int auto_increment, primary key(id));
create table state_sample_attribute (
state_sample_id int,
attribute_id int,
primary key(state_sample_id, attribute_id),
foreign key (state_sample_id) references state_sample(id) on update cascade,
foreign key (attribute_id) references attribute(id) on update cascade
);
create table note (
id int auto_increment,
state_sample_id int,
attribute_id int,
primary key(id),
foreign key (state_sample_id) references state_sample(id) on update cascade,
foreign key (state_sample_id, attribute_id)
references state_sample_attribute(state_sample_id, attribute_id) on update cascade
);
insert into attribute values (1);
insert into state_sample values (1);
insert into state_sample_attribute values (1, 1);
insert into note values (1, 1, 1);
Whenever I try to update the ss table, it fails:
update state_sample set id = 2;
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`demotemplate`.`note`, CONSTRAINT `note_ibfk_1` FOREIGN KEY (`ss_id`) REFERENCES `ss` (`id`) ON UPDATE CASCADE)
As far as I understand, this is what happens:
It tries to set state_sample.id = 2.
It sees the cascade to note and tries to update note.state_sample_id.
However, note.state_sample_id is also involved in the foreign key to to state_sample_attribute(state_sample_id, attribute_id), so it goes to check whether that's still valid.
As state_sample_attribute.state_sample_id has not yet been updated, the constraint fails.
Is my assumption correct? And if so, is there a way to work this around?

Answer 1

Give the ssa table its own id primary key, and use that in the foreign key in notes, rather than referencing the ss_id and a_id columns.
create table ssa (
id int auto_increment,
ss_id int,
a_id int,
primary key (id),
unique key (ss_id, a_id),
foreign key (ss_id) references ss(id) on update cascade,
foreign key (a_id) references a(id) on update cascade);
create table note (
id int auto_increment,
ss_id int,
ssa_id int,
primary key(id),
foreign key (ss_id) references ss(id) on update cascade,
foreign key (ssa_id) references ssa(id) on update cascade);
Now you don't have the redundant dependency.
It's also not clear that note needs ss_id at all, since it's redundant with the related ssa row.

Answer 2

Try
DISABLE KEYS
or
SET FOREIGN_KEY_CHECKS=0;
make sure to turn it on
SET FOREIGN_KEY_CHECKS=1;
after.

Answer 3

What solved it in the end is dropping the extra FK:
alter table note drop foreign key (state_sample_id) references state_sample(id);