Using primary key & foreign key to build ltree

Question

I am pretty new to postgres & especially new to ltree. Searching the web for ltree brought me to examples where the tree was build by chaining characters. But I want to use the primary key & foreign key.

Therefore I build the following table:

create table fragment(
        id serial primary key,
        description text,
        path ltree
    );
create index tree_path_idx on fragment using gist (path);

Instead of A.B.G I want to have 1.3.5. A root in the examples online is added like so:

insert into fragment (description, path) values ('A', 'A');

Instead of A I want to have the primary key (which I don't know at that moment). Is there a way to do that?

When adding a child I got the same problem:

insert into tree (letter, path) values ('B', '0.??');

I know the id of the parent but not of the child that I want to append.

Is there a way to do that or am I completey off track?

Thank you very much!

Answer 1

You could create a trigger which modifies path before each insert. For example, using this setup:

DROP TABLE IF EXISTS fragment;
CREATE TABLE fragment(
    id serial primary key
    , description text
    , path ltree
);
CREATE INDEX tree_path_idx ON fragment USING gist (path);

Define the trigger:

CREATE OR REPLACE FUNCTION before_insert_on_fragment()
RETURNS TRIGGER LANGUAGE plpgsql AS $$
BEGIN
    new.path := new.path ||  new.id::text;
    return new;
END $$;

DROP TRIGGER IF EXISTS before_insert_on_fragment ON fragment;
CREATE TRIGGER before_insert_on_fragment
BEFORE INSERT ON fragment
FOR EACH ROW EXECUTE PROCEDURE before_insert_on_fragment();

Test the trigger:

INSERT INTO fragment (description, path) VALUES ('A', '');
SELECT * FROM fragment;
-- | id | description | path |
-- |----+-------------+------|
-- |  1 | A           |    1 |

Now insert B under id = 1:

INSERT INTO fragment (description, path) VALUES ('B', (SELECT path FROM fragment WHERE id=1));
SELECT * FROM fragment;

-- | id | description | path |
-- |----+-------------+------|
-- |  1 | A           |    1 |
-- |  2 | B           |  1.2 |

Insert C under B:

INSERT INTO fragment (description, path) VALUES ('C', (SELECT path FROM fragment WHERE description='B'));
SELECT * FROM fragment;

-- | id | description |  path |
-- |----+-------------+-------|
-- |  1 | A           |     1 |
-- |  2 | B           |   1.2 |
-- |  3 | C           | 1.2.3 |