Re: A form of inheritance with PostgreSQL

Hello, and thank you to Steven and everyone else that submitted input on this issue.<br /><br /> After reading a few
moremethods of doing things, I went with the simplest one, as 1. time is of the essence, and 2. I'm stuck with
PostgreSQL7.1 on the server I have to develop for.<br /><br /> I set the primary key of the parent class to a serial.
Childrenhave an integer column with constraints as the primary key and foreign key to the parent primary key column.<br
/><br/> Thanks again,<br /><br /> - Greg<br /><br /> Steve Midgley wrote: <blockquote
cite="mid20070309163840.66FFB9FBC7D@postgresql.org"type="cite">Hi Greg, <br /><br /> While not in a C++ framework, you
mightfind 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: <br /><br /><a class="moz-txt-link-freetext"
href="http://wiki.rubyonrails.org/rails/pages/SingleTableInheritance">http://wiki.rubyonrails.org/rails/pages/SingleTableInheritance</a><br
/><br/> It's based on Martin Fowler's Patterns of Enterprise Architecture book - please find references to his original
patternshere: <br /><br /><a class="moz-txt-link-freetext"
href="http://www.martinfowler.com/eaaCatalog/singleTableInheritance.html">http://www.martinfowler.com/eaaCatalog/singleTableInheritance.html</a><br
/><br/> The key, I believe, is simply adding a "type" and a "parent_id" to the "class" table, so you can model all your
typesand their hierarchical relations. Fowler's diagram is pretty clear. I think then you would store the data in
anothertable (or tables) and link into this inheritance structure to establish ancestry for any piece of data (some
peopletry to store the data in this table too, but I think that's a mistake personally). <br /><br /> If I understand
whatyou're trying to do, you can use this design pattern in your application language to implement an inheritance
schemewithout any special database features (i.e. in a SQL-standard manner). <br /><br /> I hope this is helpful, <br
/><br/> Steve <br /><br /><br /><br /> At 12:28 AM 3/9/2007, <a class="moz-txt-link-abbreviated"
href="mailto:pgsql-sql-owner@postgresql.org">pgsql-sql-owner@postgresql.org</a>wrote: <br /><blockquote
type="cite">Date:Thu, 08 Mar 2007 13:01:51 -0500 <br /> From: Greg Toombs <a class="moz-txt-link-rfc2396E"
href="mailto:greg.toombs@bluebottle.com"><greg.toombs@bluebottle.com></a><br/> To: <a
class="moz-txt-link-abbreviated"href="mailto:pgsql-sql@postgresql.org">pgsql-sql@postgresql.org</a><br /> Subject: A
formof inheritance with PostgreSQL <br /> Message-ID: <a class="moz-txt-link-rfc2396E"
href="mailto:45F04F8F.8030800@bluebottle.com"><45F04F8F.8030800@bluebottle.com></a><br/><br />  Hello. <br /><br
/>I'm trying to figure out how to nicely implement a C++ class-like system <br /> with PostgreSQL. Consider the
following:<br /><br /> Tables Fruit, Apple, Orange <br /><br /> I want to design the foreign key scheme such that there
arerelations <br /> between fruit and apple, and fruit and orange, that imply that apple is a <br /> fruit, and orange
isa fruit. <br /><br /> I don't want to eliminate the existence of Apple and Orange tables, <br /> because there will
becolumns specific to both Apple and Orange; if I <br /> include these columns in Fruit, then if Fruit is an Orange,
theApple <br /> columns will be needlessly present in Apple rows. <br /><br /> The different ways of implementing this
schemethat I've thought of (some <br /> uglier than others): <br /><br /> - Have Fruit contain foreign keys to both
Appleand Orange, and write a <br /> check constraint in Fruit specifying that exactly one of (Apple FK, <br /> Orange
FK)needs to be non-null. The disadvantage of this method is that <br /> it isn't exactly loosely coupled. For every
otherfruit type table I <br /> implemented I'd have to go back and add a foreign key in Fruit. <br /><br /> - Have a
foreignkey in Apple to Fruit, and in Orange to Fruit; then <br /> somehow create a constraint that imposes uniqueness
onthe union of <br /> foreign keys in both Apple and Orange. To figure out what type of fruit a <br /> Fruit row is,
runa query for foreign keys in Orange and Apple matching <br /> the primary key of Fruit. You'd also want to somehow
createa constraint <br /> that the result of this query should always return exactly one row <br /> (perhaps with a
trigger?)<br /><br /> Any advice will be appreciated! As I'm relatively new to Postgre, I might <br /> need some help
withthe actual implementation as well. <br /><br /> Thank you. <br /><br /> - Greg <br /></blockquote><br /><br />
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