How to make a foreign key with a constraint on the referenced table in PostgreSQL

Question

Suppose I have the following tables

CREATE TABLE plugins (
id int primary key,
type text);

insert into plugins values (1,\'matrix\');
insert into plugins va         


        

Answer 1

You can use a CHECK constraint for this. You can't put a query in a CHECK constraint but you can call a function; so, we build a simple function that tells us if a pluginid is a matrix:

create or replace function is_matrix(int) returns boolean as $$
    select exists (
        select 1
        from plugins
        where id   = $1
          and type = 'matrix'
    );
$$ language sql;

and wrap that in a CHECK constraint:

alter table matrix_params add constraint chk_is_matrix check (is_matrix(pluginid));

Then:

=> insert into matrix_params values (1,1);
=> insert into matrix_params values (2,3);
ERROR:  new row for relation "matrix_params" violates check constraint "chk_is_matrix"

And the FK takes care of referential integrity and cascades.