Hierarchy trees in database and web app - postgresql

Question

I want to create web app which will use tree data structures. Users will be able to create, update and delete trees. I have the following table in PostgreSQL called nodes in database:
id INTEGER PRIMARY KEY,
name VARCHAR(50) NOT NULL UNIQUE,
parent_id INTEGER NULL REFERENCE nodes(id)
Getting data
I want to get data in the following form:
id | name | children
---|------|--------------
1 | a | [2,3]
2 | b | []
3 | c | [4]
4 | d | []
I created query which returns data in form
id | name | parent_id
---|------|--------------
1 | a |
2 | b | 1
3 | c | 1
4 | d | 3
And here is code:
WITH RECURSIVE nodes_cte(id, name, parent_id, level) AS (
SELECT nodes.id, nodes.name, nodes.parent_id, 0 AS level
FROM nodes
WHERE name = 'a'
UNION ALL
SELECT nodes.id, nodes.name, nodes.parent_id, level+1
FROM nodes
JOIN nodes_cte
ON nodes_cte.id = nodes.parent_id
)
SELECT * FROM nodes_cte;
Can I change SQL code to get what I want or should I do that in app??
Inserting data
I want to know what are the ways to insert data into the table. I think that following approach will work for me:
create sequence in database
increase sequence for number of elements in tree
manually compute ids in app and insert elements in the table
Are there better ways?

Answer 1

CREATE TABLE nodes
( id INTEGER PRIMARY KEY
, name VARCHAR(50) NOT NULL UNIQUE
, parent_id INTEGER NULL REFERENCES nodes(id)
);
-- I created query which returns data in form
INSERT INTO nodes(id,name,parent_id)VALUES
( 1 , 'a' , NULL)
,( 2 , 'b' , 1)
,( 3 , 'c' , 1)
,( 4 , 'd' , 3)
;
SELECT p.id, p.name
, array_agg(c.id) AS children
FROM nodes p
LEFT JOIN nodes c ON c.parent_id = p.id
GROUP BY p.id, p.name
;
Result:
id | name | children
----+------+----------
1 | a | {2,3}
2 | b | {NULL}
3 | c | {4}
4 | d | {NULL}
(4 rows)
Extra: using generate_series() to insert a bunch of records. Each record having id/3 as parent, (except when zero).
INSERT INTO nodes(id,name,parent_id)
SELECT gs, 'zzz_'|| gs::text, NULLIF(gs/3 , 0)
FROM generate_series ( 5,25) gs
;
INSERTING/UPDATING DATA
Normally, your front-end should not mess with sequences, but leave that to the DBMS. You already have a UNIQUE constraint on name, because it is a natural key . So, your front-end should use that key to address rows in the nodes table, like in:
CREATE TABLE nodes2
( id SERIAL NOT NULL PRIMARY KEY
, name VARCHAR(50) NOT NULL UNIQUE
, parent_id INTEGER NULL REFERENCES nodes(id)
);
INSERT INTO nodes2(name,parent_id)
SELECT 'Omg_'|| gs::text, NULLIF(gs/3 , 0)
FROM generate_series ( 1,15) gs
;
PREPARE upd (text, text) AS
-- child, parent
UPDATE nodes2 c
SET parent_id = p.id
FROM nodes2 p
WHERE p.name = $2 -- parent
AND c.name = $1 -- child
;
EXECUTE upd( 'Omg_12', 'Omg_11');
EXECUTE upd( 'Omg_15', 'Omg_11');
Result:
CREATE TABLE
INSERT 0 15
PREPARE
UPDATE 1
UPDATE 1
id | name | children
----+--------+-----------
1 | Omg_1 | {3,4,5}
2 | Omg_2 | {6,7,8}
3 | Omg_3 | {9,10,11}
4 | Omg_4 | {13,14}
5 | Omg_5 | {NULL}
6 | Omg_6 | {NULL}
7 | Omg_7 | {NULL}
8 | Omg_8 | {NULL}
9 | Omg_9 | {NULL}
10 | Omg_10 | {NULL}
11 | Omg_11 | {15,12}
12 | Omg_12 | {NULL}
13 | Omg_13 | {NULL}
14 | Omg_14 | {NULL}
15 | Omg_15 | {NULL}
(15 rows)