Manage revisions in postgresql - function

Question

I am currently trying to manage revisions of a data set in a postgreSql database. The table I would like to use has the following structure:
CREATE TABLE dataset (
id BIGSERIAL PRIMARY KEY,
revision INTEGER NOT NULL,
object_id BIGINT NOT NULL
);
The id field is a unique auto-increment identifier. The object_id should be the identifier for a object, while revision keeps track of the revisions:
id | object_id | revision
-------------------------
1 | 1 | 1
2 | 2 | 1
3 | 1 | 2
4 | 1 | 3
5 | 3 | 1
6 | 4 | 1
What I now need is a function, that:
Sets a auto-increment object_id and sets revision to 1, if no object_id is provided.
Sets a auto-increment revision for this object_id, if an object_id is provided.
I already found this answer, but this does not really solve the problem of creating consecutive revisions for a object_id and it does not solve the problem of auto creating consecutive object_ids.
EDIT:
I would do something like the following, but this doesn't feel very comfortable:
CREATE OR REPLACE FUNCTION update_revision() RETURNS TRIGGER LANGUAGE plpgsql AS
$$
BEGIN
IF tg_op='INSERT' THEN
IF NEW.object_id != NULL THEN
NEW.object_id = SELECT nextval(object_id_seq_id);
NEW.revision = 1;
ELSE
NEW.revision = SELECT MAX(revision)+1 FROM dataset WHERE spot_id = NEW.spot_id;
END IF;
END IF;
RETURN NEW;
END;
$$;
CREATE TRIGGER update_revision BEFORE INSERT OR UPDATE ON dataset
FOR EACH ROW EXECUTE PROCEDURE update_revision();

Answer 1

Make (object_id, revision) unique. BTW why aren't they the primary key?
create table dataset (
id bigserial primary key,
object_id bigint not null,
revision integer not null,
unique (object_id, revision)
);
create or replace function include_revision (_object_id integer)
returns dataset as $$
with object_id as (
select coalesce(max(object_id), 0) + 1 as object_id
from dataset
), revision as (
select coalesce(max(revision), 0) + 1 as revision
from dataset
where object_id = _object_id
)
insert into dataset (object_id, revision)
select
coalesce(_object_id, (select object_id from object_id)),
(select revision from revision)
returning *
;
$$ language sql;
object_id is set to coalesce(_object_id, (select object_id from object_id)), that is, only if _object_id is null it will use the calculated max(object_id)
Testing:
select include_revision(null);
include_revision
------------------
(1,1,1)
select include_revision(1);
include_revision
------------------
(2,1,2)
select include_revision(null);
include_revision
------------------
(3,2,1)