using foreign key value directly in INSERT INTO statement - sql

Question

is it possible to use the value of the foreign key directly with an INSERT INTO statement? I am using Postgresql and the tables are consttructed as follows:
CREATE TABLE public.sensors
(
name character varying(100) COLLATE pg_catalog."default",
description text COLLATE pg_catalog."default",
id integer NOT NULL DEFAULT nextval('sensors_id_seq'::regclass),
CONSTRAINT sensors_pkey PRIMARY KEY (id)
)
WITH (
OIDS = FALSE
)
TABLESPACE pg_default;
ALTER TABLE public.sensors
OWNER to postgres;
Now I also have another table, defined as:
CREATE TABLE public.testmap
(
sensor_id integer NOT NULL,
"timestamp" timestamp with time zone NOT NULL,
value "char" NOT NULL,
CONSTRAINT ragmap_sensor_id_fkey FOREIGN KEY (sensor_id)
REFERENCES public.sensors (id) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE NO ACTION
)
WITH (
OIDS = FALSE
)
TABLESPACE pg_default;
ALTER TABLE public.ragmap
OWNER to postgres;
Now, I try to insert a record directly into the testmap table as:
INSERT INTO testmap (sensor_id, timestamp, value) VALUES (1, NOW(), 'r')
I have a record inserted into the sensors table where the id is 1. However, when I try the INSERT INTO operation, I get:
DETAIL: Key (sensor_id)=(1) is not present in table "sensors".
Is there a way to use the INSERT INTO with the foreignh key directly without having to resort to another Select for the relevant row selection in the sensors table?

Answer 1

Your code doesn't even work. The column id is specified more than once for sensors.
I don't recommend having a character column as a primary key. If you do so, you should be explicit about your types:
INSERT INTO testmap (sensor_id, timestamp, value)
VALUES ('1', NOW(), 'r');
The problem is that your foreign key reference is a number but the primary key is an integer.
Instead, define the primary key to be a number:
CREATE TABLE public.sensors (
id serial primary key,
name character varying(100) COLLATE pg_catalog."default",
description text COLLATE pg_catalog."default",
id integer NOT NULL DEFAULT nextval('sensors_id_seq'::regclass),
CONSTRAINT sensors_pkey PRIMARY KEY (id)
)