If anyone is interested, my current solution for one case is:
1. add an Integer field listIndex to the item object
2. make the item object PersistenceBrokerAware, and in the
beforeUpdate() method
// update the position of this entry in the list
if (this.parent != null) {
this.listIndex = new
Integer(this.parent.getItemList().indexOf(this));
} else {
this.listIndex = null;
}
3. to the parent collection-descriptor decalaration, add <orderby
name="listIndex"/>
Possible issues:
1. If the order of items in a list is changed, then _all_ the item
objects must be updated so their indexes are stored.
As always, suggestions are welcome.
Thanks,
Rob :)
> -----Original Message-----
> From: Coup, Robert Muir [mailto:[EMAIL PROTECTED]
> Sent: Wednesday, 14 January 2004 12:09 p.m.
> To: [EMAIL PROTECTED]
> Subject: Ordered Collections, Lists, DList
>
>
> Hi all,
>
> What is the best way of storing an ordered list without using
> Dlist which requires additional tables & SQL lookups?
>
> OrderBy works great if the positioning has some underlying
> field (eg. Object id, name) but if the only thing specifying
> the position is the list index then it doesn't work too well.
>
> One option is to link a fake field-descriptor to the item's
> position in a list and use orderby... Is this what other
> people are doing?
>
> Another is to create a new ManageableCollection that is
> linked somehow to a database field which it stores indexes
> in... but that breaks the idea that the mapping is in the repository.
>
> Another is to do something like...
> <collection-descriptor
> name="myList"
> element-class-ref="com.example.MyObject"
> collection-class="com.example.MyMagicList"
> index-field="listIndexField"
> />
> Then store the index of each item in the list in the database
> field specified by index-field. When you re-instantiate it is
> effectively an orderBy. A ManageableList interface extending
> ManageableCollection may be needed which knows about and uses
> the index-field attribute...
> And it probably needs to ignore/complain about additional
> orderBy fields...
>
> Is this a good idea? Something worth doing?
> Or am I missing the obvious?
>
> Thanks again,
> Rob :)
>
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