Relationship with unique: true does not work in Postgres

[HarleySalas]

Relationship with unique: true does not work in Postgres. (Untested in mongodb) beta.30, db-postgres

to reproduce, create a collection which has a Field of type relationship, setting unique: true and proceed to create multiple documents, linking to the same relation.

In the following repo, you can observe it in the posts collection, where I attempt to make the Post.author unique, but I am able to create as many Post documents as I wish, with the same user being related to it. https://github.com/HarleySalas/payload-3-reproductions/tree/unique-relationship