Exclusion constraint on recursive foreign key (on the example of a Tree)

Question

I have the table with recursive foreign key. It's just hierarchical tree structure:

CREATE TABLE tree (
    id              INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    value           INTEGER NOT NULL,
    tree_id         SMALLINT NOT NULL CHECK (tree_id > CAST(0 AS SMALLINT)),
    parent_id       INTEGER REFERENCES tree ON DELETE RESTRICT ON UPDATE RESTRICT,

    EXCLUDE (tree_id WITH =) WHERE (parent_id IS NULL) -- allow only one root within tree
);

How to reject inserting records with non-null parent_id and different tree_id? I.e. foreign key parent_id can have reference only to primary key id within the same tree_id.

Answer 1

Use a composite foreign key:

(tree_id, parent_id) REFERENCES (tree_id, id)

You will need to first change the PK or add a UNIQUE constraint on (tree_id, id)