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From Leszek Gawron <lgaw...@mobilebox.pl>
Subject OT: O/R collection mapping [was: SQLTransformer]
Date Tue, 06 Sep 2005 07:41:13 GMT
Antonio Gallardo wrote:
> Using "orderby" attribute - 
> http://db.apache.org/ojb/docu/guides/repository.html#collection-descriptor
> There is no need of an extra field.
you can do it same way in hibernate. see: 
http://www.hibernate.org/hib_docs/v3/reference/en/html_single/#collections-sorted

<quote>
  If you want the database itself to order the collection elements use 
the order-by attribute of set, bag  or map mappings. This solution is 
only available under JDK 1.4 or higher (it is implemented using 
LinkedHashSet or LinkedHashMap). This performs the ordering in the SQL 
query, not in memory.

<set name="aliases" table="person_aliases" order-by="lower(name) asc">
     <key column="person"/>
     <element column="name" type="string"/>
</set>
</quote>

Moreover you can use a TreeSet that allows you to implement sorting not 
available to SQL (via use of Comparable interface).

Still your bean needs to have properties that define the order.

Let me give you an example that advocates the use of index field.
- a Person class (firstName, lastName, age)
- Marathon class:
   - name
   - place
   - date
   - thoseWhoFinishedTheRace collection which is a sorted list of those 
Persons who managed to get to the finish line. The list is sorted in an 
order of finishing so thoseWhoFinishedTheRace().get( 0 ) is the winner.

In this example the collection is not _sorted_ - it is _ordered_ with 
information not available to Person bean. The use of index field is 
mandatory - it's not bad database design.

> In my case, I met the Unsupported Operation Exception in jxpath using 
> @direction="save" not while loading a not with a repeater, but using a 
> cform multivalue field.
Could you give me some test case? I tried to stay out of MultiValueField 
binding as it always rewrites the whole collection which causes multiple 
deletes/inserts.

-- 
Leszek Gawron                                      lgawron@mobilebox.pl
IT Manager                                         MobileBox sp. z o.o.
+48 (61) 855 06 67                              http://www.mobilebox.pl
mobile: +48 (501) 720 812                       fax: +48 (61) 853 29 65

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