Foreign keys + table inheritance in PostgreSQL?

Question

I have three tables: organization, organization_teams and org_users. Here organization_teams is inherited from organization. So suppose if a record is added in organizations_teams it will get the organization table id as value for id column in organization_teams.

org_users has foreign key on id column of organization. Now when I try to insert data in org_users it giving me error as below

insert or update on table "org_users" violates foreign key constraint "org_users_organizations"
DETAIL:  Key (org_id)=(12) is not present in table "organizations"

Why?

Answer 1

It's covered in the user manual.

The short version: you can use foreign keys, or table inheritance, but not both. This isn't inherently impossible, it's just that it's technically quite difficult to implement unique indexes that span inherited tables in PostgreSQL in a fast, reliable manner. Without that, you can't have a useful foreign key. Nobody's successfully implemented it well enough for a patch adding support to be accepted into PostgreSQL yet.

A foreign key can point to a table that is part of an inheritance hierarchy, but it'll only find rows in that table exactly. Not in any parent or child tables. To see which rows the foreign key sees, do a SELECT * FROM ONLY thetable. The ONLY keyword means "ignoring inheritance" and that's what the foreign key lookup will do.

Answer 2

The only workaround i have found is:

1. create function returning trigger for checking existence of some id in inherited table
2. create a constraint trigger instead of FK

Answer 3

Completely agree with @Craig Ringer, we can't use foreign keys along with inheritance.

But if we are sure that we are going to insert correct data, and we want to use foreign key in org_users, we can do one thing.

We can create a child table of org_users say org_users_child (with no foreign key) as below

CREATE TABLE org_users_child () INHERITS (org_users);

Now we can insert data in this child table.

Now if we query over org_users table, we can find desired results.