Foreign Key referencing inherited table - postgresql

Question

I have the following tables:
CREATE TABLE mail (
id serial,
parent_mail_id integer,
...
PRIMARY KEY (id),
FOREIGN KEY (parent_mail_id) REFERENCES mail(id),
...
);
CREATE TABLE incoming (
from_contact_id integer NOT NULL REFERENCES contact(id),
...
PRIMARY KEY (id),
---> FOREIGN KEY (parent_mail_id) REFERENCES mail(id), <---
...
) INHERITS(mail);
CREATE TABLE outgoing (
from_user_id integer NOT NULL REFERENCES "user"(id),
...
PRIMARY KEY (id),
--> FOREIGN KEY (parent_mail_id) REFERENCES mail(id), <--
...
) INHERITS(mail);
incoming and outgoing inherit from mail and define their foreign keys (and primary keys) again, as they are not automatically inherited.
The problem is:
If I'd insert an incoming mail, it is not possible to reference it from the outgoing table as the foreign key only works with the super table (mails).
Is there a workaround for that?

Answer 1

PostgreSQL 9.3 docs:
A serious limitation of the inheritance feature is that indexes
(including unique constraints) and foreign key constraints only apply
to single tables, not to their inheritance children. This is true on
both the referencing and referenced sides of a foreign key constraint.
Thus, in the terms of the above example:
If we declared cities.name to be UNIQUE or a PRIMARY KEY, this would not stop the capitals table from having rows with names
duplicating rows in cities. And those duplicate rows would by default
show up in queries from cities. In fact, by default capitals would
have no unique constraint at all, and so could contain multiple rows
with the same name. You could add a unique constraint to capitals, but
this would not prevent duplication compared to cities.
Similarly, if we were to specify that cities.name REFERENCES some other table, this constraint would not automatically propagate to
capitals. In this case you could work around it by manually adding the
same REFERENCES constraint to capitals.
Specifying that another table's column REFERENCES cities(name) would allow the other table to contain city names, but not capital
names. There is no good workaround for this case.
These deficiencies will probably be fixed in some future release, but
in the meantime considerable care is needed in deciding whether
inheritance is useful for your application.
And not really a workaround, so maybe make mails a non-inherited table, and then separate incoming_columns and outgoing_columns for their respective extra columns, with the mail id as both their primary and foreign key. You can then create a view outgoing as mail INNER JOIN outgoing_columns, for example.

Answer 2

You may use a constraint trigger
CREATE OR REPLACE FUNCTION mail_ref_trigger()
RETURNS trigger AS
$BODY$
DECLARE
BEGIN
IF NOT EXISTS (
SELECT 1 FROM mail WHERE id = NEW.parent_mail_id
) THEN
RAISE foreign_key_violation USING MESSAGE = FORMAT('Referenced mail id not found, mail_id:%s', NEW.parent_mail_id);
END IF;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
CREATE CONSTRAINT TRIGGER mail_fkey_trigger
AFTER UPDATE OR INSERT ON incoming
DEFERRABLE
FOR EACH ROW EXECUTE PROCEDURE mail_ref_trigger();