Can PostgreSQL JOIN on jsonb array objects? - sql

Question

I am considering switching to PostgreSQL, because of the JSON support. However, I am wondering, if the following would be possible with a single query:
Let's say there are two tables:
Table 1) organisations:
ID (INT) | members (JSONB) |
------------+---------------------------------------------------------|
1 | [{ id: 23, role: "admin" }, { id: 24, role: "default" }]|
2 | [{ id: 23, role: "user" }]
Table 2) users:
ID (INT) | name TEXT | email TEXT |
------------+-----------+---------------|
23 | Max | max#gmail.com |
24 | Joe | joe#gmail.com |
Now I want to get a result like this (all i have is the ID of the organisation [1]):
ID (INT) | members (JSONB) |
------------+--------------------------------------------------------|
1 | [{ id: 23, name: "Max", email: "max#gmail.com", role:
"admin" },
{ id: 24, name: "Joe", email: "joe#gmail.com ", role:
"default" }]
(1 row)
I know this is not what JSONB is intended for and that there is a better solution for storing this data in SQL, but I am just curious if it would be possible.
Thanks!

Answer 1

Yes it is possible to meet this requirement with Postgres. Here is a solution for 9.6 or higher.
SELECT o.id, JSON_AGG(
JSON_BUILD_OBJECT(
'id' , u.id,
'name' , u.name,
'email' , u.email,
'role' , e.usr->'role'
)
)
FROM organisations o
CROSS JOIN LATERAL JSONB_ARRAY_ELEMENTS(o.data) AS e(usr)
INNER JOIN users u ON (e.usr->'id')::text::int = u.id
GROUP BY o.id
See this db fiddle.
Explanation :
the JSONB_ARRAY_ELEMENTS function splits the organisation json array into rows (one per user) ; it is usually used in combination with JOIN LATERAL
to join the users table, we access the content of the id field using the -> operator
for each user, the JSONB_BUILD_OBJECT is used to create a new object, by passing a list of values/keys pairs ; most values comes from the users table, excepted the role, that is taken from the organisation json element
the query aggregates by organisation id, using JSONB_AGG to generate a json array by combining above objects
For more information, you may also have a look at Postgres JSON Functions documentation.

Answer 2

There might be more ways to do that. One way would use jsonb_to_recordset() to transform the JSON into a record set you can join. Then create a JSON from the result using jsonb_build_object() for the individual members and jsonb_agg() to aggregate them in a JSON array.
SELECT jsonb_agg(jsonb_build_object('id', "u"."id",
'name', "u"."name",
'email', "u"."email",
'role', "m"."role"))
FROM "organisations" "o"
CROSS JOIN LATERAL jsonb_to_recordset(o."members") "m" ("id" integer,
"role" text)
INNER JOIN "users" "u"
ON "u"."id" = "m"."id";
db<>fiddle
What functions are available in detail depends on the version. But since you said you consider switching, assuming a more recent version should be fair.