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Joao Morais · Guest

Hello,

I am designing an OPF and an issue was raised. Lets say that I have this
model:

TContact
+-TPerson
+-TCompany

TInvoice
Date: TDate;
Client: TContact;

My TInvoice storage will be at least something like this:

create table Invoice (
Id: Integer;
Date: Date;
ClientId: Integer);

But what about the client class? I can: create another field as a class
reference; or I can create a join between the invoice and the contact
tables in order to retrieve the client class.

Perhaps you say: the id is a unique value inside the whole database. I
agree and like this approach, but (I think) I will need an "instances"
table that will need to be queried or joined, in order to know where I
will find an instance -- in my sample, the Client instance.

Ok, I hope you got what I mean, and I'd like to know your experience in
this area, if you:

- have an instances table and uses join, or another query to know a
class of an Id;

- don't have an instances table but a class reference per table:
create table Invoice (
Id: Integer;
ClassId: Integer;
Date: Date;
ClientId: Integer);
so you query the Contact table (or create a join) to know the class;

- have one class id per object id:
create table Invoice (
Id: Integer;
ClassId: Integer;
Date: Date;
ClientClassId: Integer;
ClientId: Integer);

- another approach?

Thanks,
--
Joao Morais