Postgres unique constraint on groups of rows - postgresql

Question

I'm using postgresql 10.12
I have labeled entities. Some are standard, some are not. Standard entities are shared among all users, whereas not standard entities are user owned. So let's say I have a table Entity with a text column Label, and a column user_id which is null for standard entities.
CREATE TABLE Entity
(
id uuid NOT NULL PRIMARY KEY,
user_id integer,
label text NOT NULL,
)
Here is my constraint : two not standard entities belonging to different users can have the same label. Standard entities labels are unique, and entities of a given users have unique labels. The hard part is: a label must be unique within a group of standard entities + a given user's entities.
I'm using sqlAlchemy, here is the constraints I've made so far:
__table_args__ = (
UniqueConstraint("label", "user_id", name="_entity_label_user_uc"),
db.Index(
"_entity_standard_label_uc",
label,
user_id.is_(None),
unique=True,
postgresql_where=(user_id.is_(None)),
),
)
My problem with this constraint is that I do not guarantee that a user entity won't have a standard entity label.
Example:
+----+---------+------------+
| id | user_id | label |
+----+---------+------------+
| 1 | null | std_ent |
| 2 | 42 | user_ent_1 |
| 3 | 42 | user_ent_2 |
| 4 | 43 | user_ent_1 |
+----+---------+------------+
This is a valid table. I want to make sure that it is not possible anymore to create an entity with label std_ent, that user 42 cannot create another entity with label user_ent_1 or user_ent_2 and that user 43 cannot create another entity with label user_ent_1.
With my current constraints, it is still possible for users 42 and 43 to create an entity with label std_ent, which is what I want to fix.
Any idea?

Answer 1

If your unique constraint(s) are doing their job of preventing users from entering duplicate labels for their own "user entities" then you can prevent them from entering the label of a "standard entity" by adding a trigger.
You create a function …
CREATE OR REPLACE FUNCTION public.std_label_check()
RETURNS trigger
LANGUAGE plpgsql
AS $function$
begin
if exists(
select * from entity
where label = new.label and user_id is null) then
raise exception '"%" is already a standard entity', new.label;
end if;
return new;
end;
$function$
;
… and then attach it as a trigger to the table
CREATE TRIGGER entity_std_label_check
BEFORE INSERT
ON public.entity FOR EACH ROW
EXECUTE PROCEDURE std_label_check()