Unique constraint on single field of a custom type in postgres

Question

I have an entity price in my schema it has an attribute amount which is of a custom type money_with_currency .

The money_with_currency is basically type (amount Big Int, currency char(3)).

The price entity belongs to a product. What I want to do is, create a unique constraint on the combination of product_id(foreign key) + currency . How can I do this?

Answer 1

Referencing a single field of a record type is a bit tricky:

CREATE TYPE money_with_currency AS (amount bigint, currency char(3));

CREATE TABLE product_price
( 
  product_id integer              not null references product, 
  price      money_with_currency  not null
);

CREATE UNIQUE INDEX ON product_price(product_id, ((price).currency));