foreign key mismatch on delete command for unrelated record

Question

I have the following 5 tables defined with a few records inserted into the 1st 4. This is using sqlite 3.7.1.7 with foreign key constaint enabled.

create table if not exists subject (id varchar(50) primary key,desc varchar(100));
insert into subject (id,desc) values ("subject1","test subject");

create table if not exists subjectlevel (id_subject_id varchar(50) references subject(id) on delete cascade, id integer not null, desc varchar(100) not null, questmcmaxselections integer not null, primary key (id_subject_id,id));

insert into subjectlevel (id_subject_id,id,desc,questmcmaxselections) values ("subject1",1,"test subject1 level 1",4);

insert into subjectlevel (id_subject_id,id,desc,questmcmaxselections) values ("subject1",2,"test subject1 level 2",4);

create table if not exists questmc (id integer primary key, text varchar(300) not null, includeallanswers int not null, subject_id varchar(50), subjectlevel_id integer, foreign key (subject_id, subjectlevel_id) references subjectlevel (id_subject_id,id) on delete cascade); 

insert into questmc (text,includeallanswers,subject_id,subjectlevel_id) values ("this is a _ question", 1, "subject1",1);

create table if not exists questmcselection (id integer primary key, text varchar(100) not null, subject_id varchar(50), subjectlevel_id integer, foreign key (subject_id, subjectlevel_id) references subjectlevel (id_subject_id,id) on delete cascade);

insert into questmcselection (text,subject_id,subjectlevel_id) values ("this is a solution","subject1",1);

create table if not exists questmc_questmcselection(id integer primary key, answer integer not null, questmc_id integer, questmcselection_id integer, subject_id varchar(50), subjectlevel_id integer, foreign key (questmc_id) references questmc(id) on delete cascade, foreign key (questmcselection_id) references questmcselection (id) on delete cascade, foreign key (subject_id,subjectlevel_id) references questmc (subject_id,subjectlevel_id) on delete cascade, foreign key (subject_id,subjectlevel_id) references questmcselection (subject_id,subjectlevel_id));

if i attempt to delete the second record in the subjectlevel table, i get a foreign key mismatch error as long as table questmc_questmcselection is defined.

sqlite> delete from subjectlevel where id=2;
Error: foreign key mismatch - "questmc_questmcselection" referencing "questmcselection"

questmc, questmcselection, and questmc_questmcselection have no related existing records that should prevent this deletion. Any idea why this error occurs?

Answer 1

This error has nothing to do with this particular subjectlevel record.

Your problem is that your tables lack the required indexes. This was not reported earlier because that DELETE statement was the first command that required SQLite to check the consistency of the database schema.

Answer 2

Based on CL's answer -

sqlite> create table parent(a);
sqlite> create table child(a, FOREIGN KEY (a) REFERENCES parent(a));
sqlite> pragma foreign_keys = ON;
sqlite> insert into parent values(3);
sqlite> insert into child values (3);
Error: foreign key mismatch - "child" referencing "parent"
sqlite> create unique index p_a on parent(a);
sqlite> insert into child values (3);
sqlite> _

From the documentation:

Usually, the parent key of a foreign key constraint is the primary key of the parent table. If [not], then the parent key columns must be collectively subject to a UNIQUE constraint or have a UNIQUE index [which uses] the collation sequences ... in the CREATE TABLE statement for the parent table.

i.e. the alternative is:

sqlite> create table parent(a, b, UNIQUE (a, b));
sqlite> create table child (x, y, FOREIGN KEY (x, y) REFERENCES parent(a, b));

(this also highlights multi-column foreign keys; they work with indexes too...)