Trying to automatically insert into a table using triggers in POSTGRESQL - postgresql

Question

I am trying to make a trigger and function that inserts into the table purchases the values which have been inserted into the table customers.
Columns of table customers
1-customer_id serial PK references customer_id in purchases
2-c_name VARCHAR
3-amount DOUBLE PRECISION
Columns of table purchases
1- customer_id serial PK 2- amount DOUBLE PRECISION
The code for the trigger and the function:
CREATE OR REPLACE FUNCTION auto_insert_purchases()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS
$body$
BEGIN
insert into purchases(customer_id,purchase) values
(NEW.customer_id,NEW.purchase);
END
$body$
CREATE TRIGGER tr_auto_insert_purchases
AFTER INSERT ON customers
EXECUTE PROCEDURE auto_insert_purchases()
As you can see its supposed to take the new row data and insert it into the table but after doing and insertion to customers like this:
insert into customers values(2,'Stewie Griffin',4.99);
I get this error message:
ERROR: null value in column "customer_id" of relation "purchases" violates not-null
constraint
DETAIL: Failing row contains (null, null).
CONTEXT: SQL statement "insert into purchases(customer_id,purchase) values
(NEW.customer_id,NEW.purchase)"
auto_insert_purchases() PL/pgSQL fonksiyonu, 3. satır, SQL ifadesi içinde
SQL state: 23502
Why does the failing row contain null? Am I using the NEW keyword incorrectly?

Answer 1

CREATE TABLE customers (
customer_id int4 NULL,
c_name varchar NULL,
amount float8 NULL
);
CREATE TABLE purchases (
customer_id int4 NULL,
amount float8 NULL
);
CREATE OR REPLACE FUNCTION auto_insert_purchases()
RETURNS trigger
LANGUAGE plpgsql
AS $function$
BEGIN
insert into purchases(customer_id, amount) values
(NEW.customer_id, NEW.amount);
return new;
END;
$function$
;
create trigger tr_auto_insert_purchases
after insert ON customers
for each row
execute procedure auto_insert_purchases();
insert into customers(customer_id, c_name, amount) values (2,'Stewie Griffin', 4.99);
select * from purchases;
-- Result:
customer_id|amount|
-----------+------+
2| 4.99|
May be you just forgot to write for each row statement after CREATE TRIGGER tr_auto_insert_purchases AFTER INSERT ON customers