Question

In PostgreSQL, what’s the simplest way to enforce more than just existence on a foreign key?

For example, given the following tables:

create table "bar"
    (
    bar_id serial primary key,
    status boolean not null
    )

create table "foo"
    (
    foo_id serial primary key,
    bar_id integer references "bar"
    )

How could foo.bar_id be constrained to only rows of bar where status is true?

I can imagine how to do it with trigger functions, but it seems like I’d need several (insert, update on foo; update, delete on bar) so I’d like to know if there’s a more convenient method, perhaps purely using constraints.

Answer 1

You could make bar.status part of the primary key of the bar table. Then you put status on the foo table as part of the FK to bar. Also put a constraint that foo.status must equal true. It’s kind of a hack, but depending on the real world context, it might make sense.