Re: First-class Polymorphic joins?

On 08/13/2015 05:59 PM, Guyren Howe wrote:

Ccing list

On Aug 13, 2015, at 17:49 , Adrian Klaver <adrian.klaver@aklaver.com> wrote:

A polymorphic join is where a fk contains not just an id but an indicator of which table it refers to.

I am pretty sure it already does that:

http://www.postgresql.org/docs/9.4/interactive/sql-createtable.html

REFERENCES reftable [ ( refcolumn ) ]

I apologize for not being clearer.

The point is that the fk in different rows can reference different tables. I might want to be able to attach a tag to a person or a blog post, say. And then I want to find all the persons and blog posts with a particular tag, in a single query.

Could you just not turn that around?:

tag
  tag_id
  tag_desc

person
  person_id
  tag_fk references tag
       
blog
  blog_id
  tag_fk references tag

The simplest implementation is to have a table reference as a first-class value I can store in a field.

--
Adrian Klaver
adrian.klaver@aklaver.com


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