postgres-json/docs/usage/multi-level.md

Multi Level Queries

Define schema, query and kotlin object

1. Schema

create table parent (
    id uuid primary key,
    name text not null
);

create table child (
    id uuid primary key,
    name text not null,
    parent_id uuid not null references parent
)

2. Insert some data for tests

insert into parent (id, name) VALUES ('379e0687-9e4a-4781-b0e9-d94a62e4261f', 'Bernard');
insert into child (id, name, parent_id) VALUES (uuid_generate_v4(), 'Noé', '379e0687-9e4a-4781-b0e9-d94a62e4261f');
insert into child (id, name, parent_id) VALUES (uuid_generate_v4(), 'John', '379e0687-9e4a-4781-b0e9-d94a62e4261f');

3. Define Model

import java.util.UUID

class Parent(val id: UUID, val name: String, val children: List<Child>)
class Child(val id: UUID, val name: String)

4. Define request function

-- resource/sql/functions/find_parent_by_id.sql
create or replace function find_parent_by_id(in _id uuid, out resource json) language plpgsql as
$$
begin
    select to_json(t) into resource
    from (
        select 
            p.*,
            json_agg(to_jsonb(c) - 'parent_id') as children   
        from parent p
        join child c on c.parent_id = p.id
        where p.id = _id
        group by p.id
    ) t;
end;
$$;

Execute the function

You just to use Requester and set the sql function name, then pass arguments.

If you need to return more than one entry, use .select() instead of .selecteOne()

See the Paginated example

import fr.postgresjson.connexion.Requester

val requester: Requester = TODO()
val result: Parent = requester
    .getFunction("find_parent_by_id")
    .selectOne("id" to "379e0687-9e4a-4781-b0e9-d94a62e4261f")

The requester create dynamically this request

select * from find_parent_by_id(_id => '379e0687-9e4a-4781-b0e9-d94a62e4261f');

Watch the underscore as prefix is added if necessary. The requester known the parameters because it parses all SQL functions and reads the names of the parameters from them.

And the SQL return is a JSON like follow:

{
  "id": "379e0687-9e4a-4781-b0e9-d94a62e4261f",
  "name": "Bernard",
  "child": [
    {
      "id": "c2d0ec81-7cac-4689-8086-2644a3b309b5",
      "name": "Noé"
    },
    {
      "id": "255d911a-0cbc-4156-bf8c-0204e89494d9",
      "name": "John"
    }
  ]
}

But the requester deserialize the result automatically into a Kotlin object with their children objects. And do that in only one request.