SQL - deleting rows from other tables with on cascade not working - sql

Question

I have 4 related tables: order_info, master_bill, master_bill_order_leg and order_leg.
MasterBill and OrderLeg have many to many relationship.
This is the quick documentation from IntelliJ for the DB tables.
Table master_bill:
create table master_bill
(
id bigint not null
primary key,
mb_no bigint not null,
created_by text not null,
updated_by text not null,
created_at timestamp with time zone default now() not null,
updated_at timestamp with time zone default now() not null
);
Table order_info:
create table order_info
(
order_id bigint not null
primary key
references public.order_t
on delete cascade,
mb_id bigint not null
references master_air_waybill
on delete cascade,
created_by text not null,
updated_by text not null,
created_at timestamp with time zone default now() not null,
updated_at timestamp with time zone default now() not null
);
Table master_bill_order_leg:
create table master_bill_order_leg
(
mb_id bigint
references master_bill
on delete cascade,
order_leg_id bigint
references order_leg
on delete cascade,
constraint master_bill_order_mb_id_order_leg_id_key
unique (mb_id, order_leg_id)
);
Table order_leg:
create table order_leg
(
id bigserial
primary key,
created_by text not null,
updated_by text not null,
created_at timestamp with time zone default now() not null,
updated_at timestamp with time zone default now() not null,
constraint order_leg_unique_c
unique (flight_id, flight_date, departure_iata, arrival_iata)
);
I have set the foreign keys and they look like this:
Table master_bill_order_leg
master_bill_order_leg_mb_id_fkey (mb_id) -> master_bill(id)
master_bill_order_leg_order_leg_id_fkey (order_leg_id) -> order_leg(id)
Table order_info
order_info_mb_id_fkey (mb_id) -> master_bill(id) ON DELETE CASCADE
order_info_order_id_fkey (order_id) -> order_t(id) ON DELETE CASCADE
I thought that if order from order_t table is deleted that all relevant rows from other tables would be deleted too, but that is not the case, only order_info row is deleted on cascade. So, I have tried with deleting a row from master_bill table:
DELETE
FROM master_bill
WHERE id = :mbId
But, then I get an error:
org.postgresql.util.PSQLException: ERROR: update or delete on table
"master_bill" violates foreign key constraint
"master_bill_order_leg_mb_id_fkey" on table
"master_bill_order_leg" Detail: Key (id)=(1076) is
still referenced from table "master_bill_order_leg".
I thought that this foreign key would delete on cascade from master_bill_order_leg table if I delete a row from master_bill, since master_bill is a parent table.
What am I doing wrong here?

Answer 1

You can use both foreign key and trigger to achieve this. (Result here)
create table master_bill
(
id bigint not null primary key,
mb_no bigint not null,
created_by text not null,
updated_by text not null,
created_at timestamp with time zone default now() not null,
updated_at timestamp with time zone default now() not null
);
create table order_info
(
order_id bigint not null primary key,
mb_id bigint not null references master_bill(id) on delete cascade,
created_by text not null,
updated_by text not null,
created_at timestamp with time zone default now() not null,
updated_at timestamp with time zone default now() not null
);
create table master_bill_order_leg
(
mb_id bigint references master_bill(id) on delete cascade,
order_leg_id bigint,
constraint master_bill_order_mb_id_order_leg_id_key unique (mb_id, order_leg_id)
);
create table order_leg
(
id bigint primary key,
created_by text not null,
updated_by text not null,
created_at timestamp with time zone default now() not null,
updated_at timestamp with time zone default now() not null
);
CREATE OR REPLACE FUNCTION public.f_delete_order_leg()
RETURNS trigger
LANGUAGE plpgsql
AS $function$
begin
delete from order_leg where id in (select order_leg_id from old_table);
return null;
end;
$function$
;
create trigger t_delete_order_leg
after delete
on master_bill_order_leg
referencing old table as old_table
for each statement
execute function f_delete_order_leg();

Answer 2

The way a CASCADE DELETE works is when you delete a record in the parent table, the related records in the child table are deleted
When you are running
DELETE
FROM order_info
WHERE order_id = :id
this is a child table (its FK references a PK in a parent table) it doesn't have any foreign keys in other tables that reference it
A simple example of how cascade deletes works is a two table database of departments and employees - one department can have many employees and this is enforced by an FK / PK relationship:
CREATE TABLE Department
(
DeptId INT PRIMARY KEY,
DeptName VARCHAR(10)
);
CREATE TABLE Employee
(
EmployeeID INT ,
EmployeeName VARCHAR(10),
DepartmentID INT,
CONSTRAINT DepartmentID_FK foreign key (DepartmentID) references Department(DeptId) ON DELETE CASCADE
);
INSERT INTO Department VALUES (1,'IT'),(2,'HR');
INSERT INTO Employee VALUES (1,'John',2),(2,'Kim',2),(3,'Sam',1);
At this point, if I run
SELECT * FROM Employee
I get 3 records.
If I delete a department from the parent table:
DELETE FROM Department WHERE DeptID = 2
and I run the select again, I now have 1 record (Sam) as the cascade delete has deleted those employees in department ID 2 (john and Kim)