Change constraint on column based on value of another
-
11-12-2020 - |
Domanda
Is it possible to change a constraint on a column in postgres based on the value of another column? E.g. (pseudocode):
CREATE TABLE transactions(
id SERIAL PRIMARY KEY,
type TXN_TYPE NOT NULL,
amount BIGINT,
. . . .,
refunded boolean DEFAULT FALSE,
refund_id DEFAULT NULL if (CONSTRAINT link_refund CHECK (refunded=TRUE))=TRUE REFERENCES transactions(id)
);
Soluzione
A foreign key can not be "conditional". The only "exception" to that rule are null
values which can't reference another table by definition.
If I understand your question correctly, you are trying to implement a constraint that says "if refunded is true
then refund_id
must reference an existing transaction".
I don't think you need the refunded
column at all. Because the flag refunded
can be derived from the value of the refund_id
using the expression: refund_id is not null
.
If you do want a column like that, just create a view with that expression.
If you do insist on having the redundant refunded
flag, you could setup a check constraint like this:
CREATE TABLE transactions(
id SERIAL PRIMARY KEY,
type TXN_TYPE NOT NULL,
amount BIGINT,
. . . .,
refunded boolean DEFAULT FALSE,
refund_id integer null REFERENCES transactions,
constraint check_refund
check ( (refunded and refund_id is not null or
(not refunded and refund_id is null) )
);