SQL Unique Constraint with columns spanning multiple tables

Question

I have a scenario where i would need to add a unique constraint using columns from 2 different tables. I have 2 tables. The first table is like,

TABLE MODEL(
    ID PRIMARY_KEY,
    KEY
)

and the second table as,

TABLE OWNERS(
    ID PRIMARY_KEY,
    MODEL_ID FOREIGN_KEY REFERENCES MODEL.ID,
    USER
)

My requirement is the combination of MODEL.KEY + OWNERS.USER should be unique. How can we achieve this?

Note: I'm using Postgres as database. There is no possibility of adding OWNERS.USER to MODEL table.

Answer 1

That should be a simple unique constraint on owners :

ALTER TABLE owners ADD UNIQUE ("user", model_id);

Avoid a name like user for columns or tables, because that is a reserved SQL keyword.