I have a rails app that uses postgresql 12. Recently, I wrote some tests and saw some strange behavior.
I have a countries table. Its schema looks like that:
qq2_test=# \d countries;
Table "public.countries"
Column | Type | Collation | Nullable | Default
-----------------------+--------------------------------+-----------+----------+---------------------------------------
id | bigint | | not null | nextval('countries_id_seq'::regclass)
domain | character varying | | not null | ''::character varying
root_city_id | bigint | | |
language_id | bigint | | not null |
currency_id | bigint | | not null |
google_tag_manager_id | character varying | | not null | ''::character varying
created_at | timestamp(6) without time zone | | not null |
updated_at | timestamp(6) without time zone | | not null |
Indexes:
"countries_pkey" PRIMARY KEY, btree (id)
"index_countries_on_domain" UNIQUE, btree (domain)
"index_countries_on_currency_id" btree (currency_id)
"index_countries_on_language_id" btree (language_id)
Foreign-key constraints:
"fk_rails_6f479b409c" FOREIGN KEY (language_id) REFERENCES languages(id)
"fk_rails_7cac1212c7" FOREIGN KEY (currency_id) REFERENCES currencies(id)
"fk_rails_beac36a0bd" FOREIGN KEY (root_city_id) REFERENCES cities(id)
Referenced by:
TABLE "country_translations" CONSTRAINT "fk_rails_0c4ee35f26" FOREIGN KEY (country_id) REFERENCES countries(id) ON DELETE CASCADE
TABLE "countries_languages" CONSTRAINT "fk_rails_556e7398aa" FOREIGN KEY (country_id) REFERENCES countries(id) ON DELETE CASCADE
TABLE "cities" CONSTRAINT "fk_rails_996e05be41" FOREIGN KEY (country_id) REFERENCES countries(id)
As you can see, I have foreign key constraints on both currency_id and language_id fields.
When I run my tests I see that there are records in that table:
qq2_test=# select * from countries;
id | domain | root_city_id | language_id | currency_id | google_tag_manager_id | created_at | updated_at
-----------+--------+--------------+-------------+-------------+-----------------------+----------------------------+----------------------------
665664142 | com | | 1019186233 | 432072940 | | 2020-10-23 06:20:49.288637 | 2020-10-23 06:20:49.288637
169150333 | by | | 1019186233 | 432072940 | | 2020-10-23 06:20:49.288637 | 2020-10-23 06:20:49.288637
(2 rows)
There are two my test records and they have language and currency references. But their tables are empty:
qq2_test=# select * from currencies;
id | name | symbol | created_at | updated_at
----+------+--------+------------+------------
(0 rows)
qq2_test=# select * from languages;
id | name | locale | image | created_at | updated_at
----+------+--------+-------+------------+------------
(0 rows)
Why does postgresql allow nonexistent references in countries table?
Assuming that you're the only person using the database (since you are talking about small, 1-2 row tests), I would guess that your Rails app (or corresponding driver) is disabling triggers or foreign key checks. It's totally possible to bypass the foreign key checks like so:
edb=# show session_replication_role ;
session_replication_role
--------------------------
origin
(1 row)
edb=# create table city (id int primary key, name text);
CREATE TABLE
edb=# select * from city;
id | name
----+------
(0 rows)
edb=# create table person (id int, name text, city int references city(id));
CREATE TABLE
edb=# insert into person values (1,'foo',1);
ERROR: insert or update on table "person" violates foreign key constraint "person_city_fkey"
DETAIL: Key (city)=(1) is not present in table "city".
edb=# set session_replication_role to replica;
SET
edb=# insert into person values (1,'foo',1);
INSERT 0 1
edb=# select * from person;
id | name | city
----+------+------
1 | foo | 1
(1 row)
edb=# select * from city;
id | name
----+------
(0 rows)
I would suggest that you temporarily set log_statement = all and run your tests again--then see in your Postgres server logs (default should be /var/log/postgresql/postgresql-12-main.log for Ubuntu) what might be disabling your foreign key constraint checks, then address your findings accordingly.