Two postgresql tables referencing each other - sql

Question

Question may be basic, I don't have any experience with databases.
I have a postgres db with some tables. Two of them are dates and accounts.
The date table has an account_id field referencing an id table in an account table and a balance field that represents the balance that account had at that date. So, many date entities may reference one account entity, many-to-one, okay.
But an account table also has an actual_date field, that must reference the date entity, with actual balance this account has. One account entity may reference one actual date entuty, but date entity can have one or zero account entities referncing it. And if it does have an account referencing it with it's actual_date, it will always be the same account, date itself referencing with account_id.
What kind of relathinship is this? Is it even possible to implement? And if it is, how do I do it?
I came up with this piece of code, but I have no clue if it does what I think it does.
CREATE TABLE accounts (
id SERIAL PRIMARY KEY,
user_id INT REFERENCES users,
actual_date_id DATE UNIQUE REFERENCES dates
);
CREATE TABLE dates (
id SERIAL PRIMARY KEY,
account_id INT REFERENCES accounts,
date DATE,
balance INT,
unconfirmed_balance INT
);
P.S. I create tables with init.sql but work with them with sqlalchemy and it would be greate if someone could also show how to define such model with it.

Answer 1

As written the SQL script would never work for two reasons:
a foreign key can only reference the primary key of a table, not any arbitrary column in it. So actual_date_id should be an integer in order to be able to reference the primary key of the dates table.
you can't reference a table that hasn't been created yet, so the foreign key between accounts and dates must be created after both tables are created.
With circular foreign keys it's usually easier to define at least one of them as deferrable, so that you can insert them without the need of e.g. an intermediate NULL value.
So something along the lines (assuming that users already exists)
CREATE TABLE accounts (
id SERIAL PRIMARY KEY,
user_id INT REFERENCES users,
actual_date_id integer UNIQUE -- note the data type
);
CREATE TABLE dates (
id SERIAL PRIMARY KEY,
account_id INT REFERENCES accounts,
date DATE,
balance INT,
unconfirmed_balance INT
);
-- now we can add the foreign key from accounts to dates
alter table accounts
add foreign key (actual_date_id)
references dates (id)
deferrable initially deferred;
It might be better to avoid the circular reference to begin with. As you want to make sure that only one "current balance" exists for each account, this could be achieved by adding a flag in the dates table and getting rid of the actual_date_id in the accounts table.
CREATE TABLE accounts (
id SERIAL PRIMARY KEY,
user_id INT REFERENCES users
);
CREATE TABLE dates (
id SERIAL PRIMARY KEY,
account_id INT REFERENCES accounts,
is_current_balance boolean not null default false,
date DATE,
balance INT,
unconfirmed_balance INT
);
-- this ensures that there is exactly one row with "is_current_balance = true"
-- for each account
create unique index only_one_current_balance
on dates (account_id)
where is_current_balance;
Before you change a row in dates to be the "current one", you need to reset the existing one to false.
Unrelated, but:
With modern Postgres versions it's recommended to use identity columns instead of serial