Referencing one object many times

Question

Hallo,
I have an m:n relationship between products and spareparts (both object tables):
create or replace type sparepart_TY as object(
sparepartid number,
designation varchar2(50)
create or replace type sparepart_NT as table of ref sparepart_TY
create or replace type product_TY as object(
productid number,
designation varchar2(50),
spareList sparepart_NT
create table spareparts of sparepart_TY
create table products of product_TY
nested table spareList store as spareList_NT_TAB
This way I can reference many spare parts to one product.
But how can I show, that one product has the same spare part several times?

Answer 1

As Chris suggests, you can
reference several times the same instance:
insert into spareparts values (101, 'SP 1');
insert into spareparts values (102, 'SP 2');
insert into products values (
  '1', 'P 1',
  sparepart_NT (
    (select ref (sp) from spareparts sp where sparepartid = 101),
    (select ref (sp) from spareparts sp where sparepartid = 102),
    (select ref (sp) from spareparts sp where sparepartid = 102)
);or redefine the type "sparepart_nt" to contain, in addition to the ref, a new attribute "quantity":
create or replace type sparepart_ref_qty_TY as object (
  sparepartref ref sparepart_TY,
  quantity number (2)
create or replace type sparepart_NT2 as table of sparepart_ref_qty_TY
create or replace type product_TY2 as object (
  productid number,
  designation varchar2(50),
  spareList sparepart_NT2
create table products2 of product_TY2
  nested table spareList store as spareList_NT_TAB2
insert into products2 values (
  '1', 'P 1',
  sparepart_NT2 (
    (sparepart_ref_qty_TY ((select ref (sp) from spareparts sp where sparepartid = 101), 1)),
    (sparepart_ref_qty_TY ((select ref (sp) from spareparts sp where sparepartid = 102), 2))
);But, I don't like REF's and object tables.
First, REF can't replace good old foreign keys. After:
delete spareparts;
we have "dangling REF's" in products (in nested table spareList).
C.J.Date in "An introduction to Database Systems" (eighth edition, 2004, chapter 26)
says that using REF's in relational databases we make "The Second Great Blunder":
- "The blunder consists of mixing pointers and relations"
- "The Second Great Blunder undermines the conceptual integrity of the relational model in numerous ways"...
"The First Great Blunder" is to equate object classes and relational variables,
simply speaking - to have object tables (tables in which each row represents an object).
Without "The First Great Blunder" (object tables) we can't make "The Second Great Blunder" (using REF's),
but we can have object tables without REF's.
The "proper way" (in Date's sense) of how to use "object-relational" features is - to use object type as domain.
Regards,
Zlatko