From: Matthias Nagel <matthias.h.nagel@gmail.com>
To: pgsql-sql@postgresql.org
Subject: Restrict FOREIGN KEY to a part of the referenced table
Date: Thu, 11 Apr 2013 09:55:20 +0200
Message-ID: <3358559.iSkkklqms2@hek506>
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Hello,

is there any best practice method how to create a foreign key that only allows values from those rows in the referenced table that fulfill an additional condition?

First I present two pseudo solutions to clarify what I would like to do. They are no real solutions, because they are neither SQL standard nor postgresql compliant. The third solution actually works, but I do not like it for reason I will explain later:

CREATE TABLE parent (
  id SERIAL,
  discriminator INT NOT NULL,
  attribute1 VARCHAR,
  ...
);


Pseudo solution 1 (with a hard-coded value):

CREATE TABLE child (
  id SERIAL NOT NULL,
  parent_id INT NOT NULL,
  attribute2 VARCHAR,
  ...,
  FOREIGN KEY ( parent_id, 42 ) REFERENCES parent ( id, discriminator )
);


Pseudo solution 2 (with a nested SELECT statement):

CREATE TABLE child (
  id SERIAL NOT NULL,
  parent_id INT NOT NULL,
  attribute2 VARCHAR,
  ...,
  FOREIGN KEY ( parent_id ) REFERENCES ( SELECT * FROM parent WHERE discriminator = 42 ) ( id )
);


Working solution:

CREATE TABLE child (
  id SERIAL NOT NULL,
  parent_id INT NOT NULL,
  parent_discriminator INT NOT NULL DEFAULT 42,
  attribute2 VARCHAR,
  ...,
  FOREIGN KEY ( parent_id, parent_discriminator ) REFERENCES parent ( id, discriminator ),
  CHECK ( parent_discriminator = 42 )
);


The third solution work, but I do not like it, because it adds an extra column to the table that always contains a constant value for the sole purpose to be able to use this column in the FOREIGN KEY clause. On the one hand this is a waste of memory and on the other hand it is not immediately obvious to an outside person what the purpose of this extra column and CHECK clause is. I am convinced that any administrator who follows me might get into problems to understand what this is supposed to be. I would like to have a more self-explanatory solution like 1 or 2.

I wonder if there is something better.

Best regards, Matthias

----------------------------------------------------------------------
Matthias Nagel
Willy-Andreas-Allee 1, Zimmer 506
76131 Karlsruhe

Telefon: +49-721-8695-1506
Mobil: +49-151-15998774
e-Mail: matthias.h.nagel@gmail.com
ICQ: 499797758
Skype: nagmat84


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