convert JSON array to PostgreSQL array for use in a join - arrays

Question

I am using Postgres 9.6, and I have the following two tables:
create table employees (
id text,
setting_id text -- references settings.id
);
create table settings (
id text,
setting_str text -- contains json string
);
insert into employees (id, setting_id) values ('e1', 's1');
insert into employees (id, setting_id) values ('e2', 's2');
insert into settings (id, setting_str)
values ('s1', '{"vehicles" : null}');
insert into settings (id, setting_str)
values ('s2', '{"vehicles" : ["Car", "Bike"]}');
Now I want to get output like:
employee_id, name, vehicles
e1, one, null
e2, two, {"Car", "Bike"}
I tried with the following query:
select e.id,
jsonb_array_elements_text(s.setting_str::jsonb #> '{vehicles}')
from employees e
join settings s on s.id = e.setting_id;
But it gives me an error:
ERROR: cannot extract elements from a scalar
Any idea how I can extract the JSON array from the text field and display it as a Postgres array of strings (not a json array, not text) in a select statement?

Answer 1

The data model is not good, so the query will be complicated.
You should use a relational model for data like this.
Here is the query with inline comments:
SELECT e.id AS employee_id,
s.id AS setting_id,
/* construct an array of all vehicles that are not NULL */
array_agg(v.p) FILTER (WHERE v.p IS NOT NULL) AS vehicles
FROM employees AS e
JOIN settings AS s ON e.setting_id = s.id
/* join with the "exploded" JSON array from s.setting_str */
LEFT JOIN LATERAL json_array_elements_text(
/* replace "null" with an empty array */
COALESCE(
(s.setting_str::json) ->> 'vehicles',
'[]'
)::json
) AS v(p) ON TRUE
GROUP BY e.id, s.id;
employee_id | setting_id | vehicles
-------------+------------+------------
e1 | s1 |
e2 | s2 | {Car,Bike}
(2 rows)