> On Aug 27, 2018, at 6:00 AM, Tobias Gierke <tobias.gie...@code-sourcery.de>
> wrote:
>
> Hi,
>
> A collegue of mine just came across a rather interesting bug in our Wicket
> application.
>
> 1. We have a simple page with a repeater (ListView) that displays a table and
> on each row, some buttons to perform actions on the item shown on this row
> (edit/delete/etc.)
> 2. The underlying data source (a database table) gets updated concurrently by
> another process running on another machine
> 3. The table is supposed to always show the latest data at the very top, so
> the page uses a LoadableDetachableModel to always hit the database on every
> request
>
> The bug:
>
> Users complained that performing actions on the data items would actually
> affect an item different from what they clicked.
>
> The explanation:
>
> Since the list model got detached at the end of the previous request,
> clicking any of the action buttons would re-populate the data model, fetching
> previously unseen rows from the database. Since (according to my
> collegue,didn't double-check) the ListView associates the item models only
> based on their list index, the action button on the very first row now all of
> a sudden referred to a database row the user didn't even know about.
>
This is exactly why ListViews should not be used to work with database data
unless you override getListItemModel() to return a model to represent the item
itself like Sven mentioned in his reply.
Here are three different ways to fix your problem from worst to best:
1. add(new AjaxButton( "delete , new EntityModel(item.getModelObject()))
Where EntityModel knows how to load the entity from the database - ie the jpa
model sven mentioned.
2. New ListView<….> {
getListItemModel(imodel list, int index) { return new
EntityModel(list.getobject().get(index)); }
This is the same as above but has the advantage of item.getmodel() returning
the better model
3. Use a RefreshingView or a DataView instead.
-Igor
> add(new AjaxButton( "delete , item.getModelObject() )
> His fix:
>
> Instead of
>
> view = new ListView<Data>("listView" , dataProvider )
> {
> @Override
> protected void populateItem(ListItem<PCAPFile> item)
> {
> add(new AjaxButton( "delete , item.getModel() ) // use model from
> ListItem (model gets detached after request)
> {
> public void onClick(AjaxRequestTarget target) {
> delete( getModelObject() );
> }
> });
> // ... more stuff
> }
> }
>
> he changed the code to read:
>
> view = new ListView<Data>("listView" , dataProvider )
> {
> @Override
> protected void populateItem(ListItem<PCAPFile> item)
> {
> add(new AjaxButton( "delete , item.getModelObject() ) // capture model
> object when constructing the button
> {
> public void onClick(AjaxRequestTarget target) {
> delete( getModelObject() );
> }
> });
> // ... more stuff
> }
> }
>
> This obviously is a rather subtle issue and - depending on the size of your
> model objects - also comes with a certain performance/memory cost because of
> the additional serialization for the model items the repeater components are
> now holding onto.
>
> Is this the recommended approach for dealing with dynamically changing data
> or is there a better way to do it ?
>
> Thanks,
> Tobias
>
>
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