Re: A form of inheritance with PostgreSQL - Mailing list pgsql-sql

Hi Greg,

While not in a C++ framework, you might find that it's not too hard to 
implement something similar in your system - It's called "Single Table 
Inheritance." References to the Ruby on Rails implementation here:

http://wiki.rubyonrails.org/rails/pages/SingleTableInheritance

It's based on Martin Fowler's Patterns of Enterprise Architecture book 
- please find references to his original patterns here:

http://www.martinfowler.com/eaaCatalog/singleTableInheritance.html

The key, I believe, is simply adding a "type" and a "parent_id" to the 
"class" table, so you can model all your types and their hierarchical 
relations. Fowler's diagram is pretty clear. I think then you would 
store the data in another table (or tables) and link into this 
inheritance structure to establish ancestry for any piece of data (some 
people try to store the data in this table too, but I think that's a 
mistake personally).

If I understand what you're trying to do, you can use this design 
pattern in your application language to implement an inheritance scheme 
without any special database features (i.e. in a SQL-standard manner).

I hope this is helpful,

Steve



At 12:28 AM 3/9/2007, pgsql-sql-owner@postgresql.org wrote:
>Date: Thu, 08 Mar 2007 13:01:51 -0500
>From: Greg Toombs <greg.toombs@bluebottle.com>
>To: pgsql-sql@postgresql.org
>Subject: A form of inheritance with PostgreSQL
>Message-ID: <45F04F8F.8030800@bluebottle.com>
>
>  Hello.
>
>I'm trying to figure out how to nicely implement a C++ class-like 
>system
>with PostgreSQL. Consider the following:
>
>Tables Fruit, Apple, Orange
>
>I want to design the foreign key scheme such that there are relations
>between fruit and apple, and fruit and orange, that imply that apple 
>is a
>fruit, and orange is a fruit.
>
>I don't want to eliminate the existence of Apple and Orange tables,
>because there will be columns specific to both Apple and Orange; if I
>include these columns in Fruit, then if Fruit is an Orange, the Apple
>columns will be needlessly present in Apple rows.
>
>The different ways of implementing this scheme that I've thought of 
>(some
>uglier than others):
>
>- Have Fruit contain foreign keys to both Apple and Orange, and write 
>a
>check constraint in Fruit specifying that exactly one of (Apple FK,
>Orange FK) needs to be non-null. The disadvantage of this method is 
>that
>it isn't exactly loosely coupled. For every other fruit type table I
>implemented I'd have to go back and add a foreign key in Fruit.
>
>- Have a foreign key in Apple to Fruit, and in Orange to Fruit; then
>somehow create a constraint that imposes uniqueness on the union of
>foreign keys in both Apple and Orange. To figure out what type of 
>fruit a
>Fruit row is, run a query for foreign keys in Orange and Apple 
>matching
>the primary key of Fruit. You'd also want to somehow create a 
>constraint
>that the result of this query should always return exactly one row
>(perhaps with a trigger?)
>
>Any advice will be appreciated! As I'm relatively new to Postgre, I 
>might
>need some help with the actual implementation as well.
>
>Thank you.
>
>- Greg