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
aperson 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