How to automatically fill columns on a row based on the foreign key being the primary key on another table - postgresql

Question

I'm working on a sql database for QGIS. I have 8 tables: 3 three of them are the base tables and the others are relational tables. One relational table,
"tabela_is_po", has 4 fields:
indice_sequencial_po (integer and Primary Key),
tema (character varying),
subtema (character varying),
designacao (character varying)
And the base table, "tabela_objecto_area", has 7 fields:
identificador integer ,
dtcc character varying(4),
planta_po boolean,
indice_sequencial_po integer (foreign key),
tema_po character varying(254),
subtema_po character varying(254),
designacao_po character varying(254)
In "tabela_objecto area", when I fill, indice_sequencial_po integer (foreign key), I want the table to be able to automatically get the data from the relational table "tabela_is_po", based on the number that is filled in indice_sequencial_po.
I've tried the trigger function but it never worked well.
tabela_objecto_area code:
CREATE TABLE pdm2019.tabela_objecto_area
(
identificador integer NOT NULL DEFAULT nextval('pdm2019.tabela_objecto_area_identificador_seq'::regclass),
dtcc character varying(4) NOT NULL DEFAULT '0101',
planta_po boolean NOT NULL,
indice_sequencial_po integer,
tema_po character varying(254),
subtema_po character varying(254),
designacao_po character varying(254),
CONSTRAINT tabela_objecto_area_pkey PRIMARY KEY (identificador)
)
WITH (
OIDS=FALSE
);
Trigger function:
CREATE OR REPLACE FUNCTION tema_e_subtema_automatico() RETURNS TRIGGER AS $tema_e_subtema_automatico$
BEGIN
IF NEW.pdm2019.tabela_objecto_area.indice_sequencial_po IS NOT NULL THEN
INSERT INTO pdm2019.tabela_objecto_area(tema_po,subtema_po,designacao_po) SELECT tema,subtema,designacao FROM pdm2019.tabela_is_po WHERE indice_sequencial_po = NEW.pdm2019.tabela_objecto_area.indice_sequencial_po;
END IF;
END;
$tema_e_subtema_automatico$ LANGUAGE plpgsql;
CREATE TRIGGER tema_e_subtema_automatico AFTER INSERT OR UPDATE ON pdm2019.tabela_objecto_area
FOR EACH ROW EXECUTE PROCEDURE tema_e_subtema_automatico();

Answer 1

You would probably be better off using a view. See https://en.wikipedia.org/wiki/Relational_model for an overview, basically each item should appear only once in the database.
For data entry, it's often easier to use a framework like django (https://www.djangoproject.com/) to create the structure of your tables and input data, which you can then view in qgis.
CREATE TABLE pdm2019.tabela_objecto_area
(
identificador integer NOT NULL DEFAULT nextval('pdm2019.tabela_objecto_area_identificador_seq'::regclass),
dtcc character varying(4) NOT NULL DEFAULT '0101',
planta_po boolean NOT NULL,
indice_sequencial_po integer,
-- tema_po character varying(254), -- use a view to populate these
-- subtema_po character varying(254), -- use a view to populate these
-- designacao_po character varying(254), -- use a view to populate these
CONSTRAINT tabela_objecto_area_pkey PRIMARY KEY (identificador)
)
WITH (
OIDS=FALSE
);
CREATE VIEW v_tabela_objecto_area as (
select a.*, b.tema_po, b.subtema_po, b.designacao_po
from tabela_objecto_area a, tabela_is_po b
where a.indice_sequencial_po = b.indice_sequencial_po);