Justin,
That's a little bit tricky since it involves some advanced
mapping capabilities from a vendor's CMP engine. It also
falls outside the realm of the EJB 2.0 specification (how
convenient!), since the spec does not define how an abstract
schema maps to an underlying persistent store. Or to be
specific in this case - how an abstract schema maps to
(potentially) multiple tables in an RDBMS.
You're basically looking for a feature that allows an entity
bean's fields to map to multiple tables. Some AppServer
vendors do provide such a feature in their CMP 2.0 engines
(atleast we do), but the caveat here is that such a feature
is vendor specific.
In Borland Enterprise Server's case, we provide the feature
by:
(a) Allowing an entity bean to map to columns in multiple
tables.
(b) Allowing the developer to override EJB-QL generated
by the CMP engine with your own SQL (which in this
case would provide the JOIN over the cross reference
table and select not only the columns from the Artist
table, but also the 'sequenceNumber' column from the
cross reference table.
If portability among AppServer vendors is a concern, then
the other options are to use straight forward JDBC (say
from a session bean or DAO) or resort to BMP.
A less elegant approach (which I dislike - personally) is
to create an entity bean that maps to your cross reference
table.
In case you're interested in how our CMP engine provides
the above mentioned features, here are a couple of links:
http://info.borland.com/techpubs/bes/entity_persistence20.html
http://info.borland.com/techpubs/bes/ejb-ql.html
-krish
Borland
> -----Original Message-----
> From: A mailing list for Enterprise JavaBeans development
> [mailto:[EMAIL PROTECTED]] On Behalf Of Justin Wesbrooks
> Sent: 02 January 2003 18:03
> To: [EMAIL PROTECTED]
> Subject: How to add an extra field with M:N relationship in CMR
>
>
> I have 2 tables with a M:N relationship and a bridge table
> that joins the
> two.
>
> Product Table Bridge Table Artist Table
>
> UPC UPC ARTISTID
> TITLE Artist ID ARTISTNAME
> ETC.... Sequence Number ETC...
>
> The "Sequence Number" field is used to determine the sequence
> that artists
> appear on the physical piece of product. When I use CMR's, I need to
> include
> "sequence number" in my ArtistEJB which is contained in a
> collection in the
> ProductEJB.
>
> Are there any examples of this? I tried searching Google but
> couldn't find
> anything.
>
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