Converting relational tree to nested json document in postgres - postgresql

Question

In PostgreSQL have a relational data model that represents the hierarchy within an organization.
create table employee (
id integer primary key,
name varchar(40) not null,
supervisor_id integer references employee
);
Only the CEO has supervisor_id=NULL. Every other employ has a supervisor with some supervisor_id.
I would like export the data as a single nested json document
{
"id": 1,
"name": "Name of company's CEO",
"supervises": [
{
"id": 2,
"name": "Name of 1st EC member",
"supervises": [ ... nested employees ... ]
},
{
"id": 3,
"name": "Name of 2nd EC member",
"supervises": [ ... nested employees ... ]
}
...
]
}
I followed the example from https://www.postgresqltutorial.com/postgresql-recursive-query/ but it only helps me to identify all the employees top down along the reporting line using the WITH RECURSIVE clause.
I know that I need to start the aggregation with the employees that have the highest depth in the tree (not just leaf nodes) and aggregate them bottom up but I did not manage to write a query that does the job.
Thanks for your help!

Answer 1

At first glance your question sounds like quite simple using a recursive query, but it is not because of the list of employees who may have the same supervisor, which requires an aggregate function in order to build the corresponding json array, but the aggregate functions are not allowed in a recursive query ...
First step - We build the json objects for every employee and the corresponding array of supervised employees when he/she is a supervisor :
SELECT s.id AS father
, CASE WHEN array_agg(e.id) = array[NULL :: integer] THEN NULL ELSE array_agg(e.id) END AS children_list
, jsonb_build_object
( 'id', s.id
, 'name', s.name
, 'supervises', array_agg(e.id)
) AS json_tree
FROM employee AS s
LEFT JOIN employee AS e
ON s.id = e.supervisor_id
GROUP BY s.id, s.name
Second step - Using a recursive query, we go through the tree of employees in a top-down approach, we assign a rank to every employee and build the corresponding jsonpath that will be used in the next step :
WITH RECURSIVE elt AS
(
SELECT s.id AS father
, CASE WHEN array_agg(e.id) = array[NULL :: integer] THEN NULL ELSE array_agg(e.id) END AS children_list
, jsonb_build_object
( 'id', s.id
, 'name', s.name
, 'supervises', array_agg(e.id)
) AS json_tree
FROM employee AS s
LEFT JOIN employee AS e
ON s.id = e.supervisor_id
GROUP BY s.id, s.name
), list (father, json_tree, children_list, rank, path) AS
(
SELECT c.father, c.json_tree, c.children_list, 1, '{}' :: text[]
FROM elt AS c
LEFT JOIN elt AS f
ON array[c.father] <# f.children_list
WHERE f.father IS NULL
UNION ALL
SELECT c.father
, c.json_tree
, c.children_list
, f.rank + 1
, f.path || array['supervises',(array_position(f.children_list, c.father)-1) :: text]
FROM list AS f
INNER JOIN elt AS c
ON array[c.father] <# f.children_list
) --, ordered_list AS (
SELECT *
FROM list
ORDER BY rank DESC
Third step - We create the aggregate version of the jsonb_set function so that to build the final jsonb data while iterating on the previous resulting list
CREATE OR REPLACE FUNCTION jsonb_set(x jsonb, y jsonb, p text[], e jsonb, b boolean)
RETURNS jsonb LANGUAGE sql AS $$
SELECT CASE WHEN x IS NULL THEN e ELSE jsonb_set(x, p, e, b) END ; $$ ;
CREATE OR REPLACE AGGREGATE jsonb_set_agg(x jsonb, p text[], e jsonb, b boolean)
( STYPE = jsonb, SFUNC = jsonb_set) ;
Final query
WITH RECURSIVE elt AS
(
SELECT s.id AS father
, CASE WHEN array_agg(e.id) = array[NULL :: integer] THEN NULL ELSE array_agg(e.id) END AS children_list
, jsonb_build_object
( 'id', s.id
, 'name', s.name
, 'supervises', array_agg(e.id)
) AS json_tree
FROM employee AS s
LEFT JOIN employee AS e
ON s.id = e.supervisor_id
GROUP BY s.id, s.name
), list (father, json_tree, children_list, rank, path) AS
(
SELECT c.father, c.json_tree, c.children_list, 1, '{}' :: text[]
FROM elt AS c
LEFT JOIN elt AS f
ON array[c.father] <# f.children_list
WHERE f.father IS NULL
UNION ALL
SELECT c.father
, c.json_tree
, c.children_list
, f.rank + 1
, f.path || array['supervises',(array_position(f.children_list, c.father)-1) :: text]
FROM list AS f
INNER JOIN elt AS c
ON array[c.father] <# f.children_list
)
SELECT jsonb_set_agg(NULL :: jsonb, path, json_tree, true ORDER BY rank ASC)
FROM list
test result in dbfiddle