Use result of postgres CTE in function - postgresql

Question

I am having difficulty using the results from a CTE in a function. Given the following Postgres table.
CREATE TABLE directory (
id SERIAL PRIMARY KEY
, name TEXT
, parent_id INTEGER REFERENCES directory(id)
);
INSERT INTO directory (name, parent_id)
VALUES ('Root', NULL), ('D1', 1), ('D2', 2), ('D3', 3);
I have this recursive CTE that returns the descendants of a directory.
WITH RECURSIVE tree AS (
SELECT id
FROM directory
WHERE parent_id = 2
UNION ALL
SELECT directory.id
FROM directory, tree
WHERE directory.parent_id = tree.id
)
The returned values are what I expect and can be made to equal an array
SELECT (SELECT array_agg(id) FROM tree) = ARRAY[3, 4];
I can use an array to select values from the table
SELECT * FROM directory WHERE id = ANY(ARRAY[3, 4]);
However, I cannot use the results of the CTE to accomplish the same thing.
SELECT * FROM directory WHERE id = ANY(SELECT array_agg(id) FROM tree);
The resulting error indicates that there is a type mismatch.
HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.
However, I am unsure how to correctly accomplish this.

Answer 1

Use:
SELECT *
FROM directory
WHERE id = ANY(SELECT unnest(array_agg(id)) FROM tree);
See detailed explanation in this answer.
Using unnest() in a subquery is a general method for dealing with arrays:
where id = any(select unnest(some_array))
Because array_agg() and unnest() are inverse operations, the query can be as simply as:
SELECT *
FROM directory
WHERE id = ANY(SELECT id FROM tree);