DataModel, SelectItem(s) Reading data twice - selection consistency

Question

Hello,
I have some concerns regarding the way how selected item in selectone_menu or command_link/command_button inside of data_table get resolved on submit.
I am very new to JSF so I might misunderstood how it works and will be grateful for your comments
Here are two samples which assume Request scope for their backing beans.
Let�s say we have a typical form with Products drop down list
When we render the form Products gets populated from List of SelectItem which is built in backing bean from database
When we submit the form the view is recreated, product list is repopulated from backing bean again and user selection is resolved
Very similar case with data_table - in fact even more confusing because it seems use row numbers. When we render it its underlying Data Model gets populated from database and each row of the table outputs lets say a "delete" button. One of "Delete" buttons is clicked - form gets submitted. View is reconstituted by repopulating DataModel and resolving row which was clicked (using button ID and Row Number if I understand it right)
Now in both cases to successfully resolve user selection JSF needs to get exactly the same data in the list or DataModel which can't not guaranteed by application. If DataModel retrieves different set of data I am afraid user click might get resolved to a wrong row
Another issue is performance. I am not totally happy with the fact that everything gets fetched twice - first time on render (expected) and second time on submit
Thank you
Alex

Answer 1

Adam,
I think it makes use of DataModel difficult and uncertain. And it is one of the most widely used scenarious - select something from a dynamically produced list.
As it stands now data_table can
1. Cause event propagation to wrong row in case of dynamic data
2. Performs very expensive data retrieval twice just so click can be resolved to row object
Storing data model in session for the time of render/submit cycle is possible but
1. Not an optioin in many cases because the model references "live" data (like JDO which holds reference to PersistenceManager) and is not serializable it is meant to be used in request and released.
2. But even if it was not an issue it is a very high price to pay in terms of transparency ease of development etc. I am not even talking about technical chalenges of purging snapshots of various data models from session
I wonder if there are any other approaches to DataModel. For example require a RowID for each row.
In most cases it is there anyway in form of primary keys or object IDs of underlying data. And in many cases it is all developers would want to get on row event - RowID. If DataModel had getSelectedRowID() method it would be all we need to go on. If people need to get an actual instance to fire event on DataModel could optionally implement resolveRow(rowId) method which will resolve selected ID to row data instance. It will also take care of double retrieve issue because you will not need to retrieve the same data second time - just selected row data.
Somehow I think in 80% cases people won't even need to get row instance in their event - just row id