Update parts of a dataset

I've got an Master and a Child table and i use an BindingSource object to join them. When I click on a row in the master table i get all its child rows. Now I want to update this rows with my dataAdapter. Now it's the problem how I only get the changes from the selection that is actually visible, because i don't want to save the other rows I maybe changed before.

I tried it like this:
Dim dvAsNew DataView(DataSet,"ID = " & id,"", DataViewRowState.ModifiedCurrent)
Dim tmpDsAs DataSet

tmpDs.Tables.Add("table")
tmpDs.Tables("table").Merge(dv.ToTable)

webservice.savedata(tmpDs)

But the result of this was that instead of updating my rows, there were new rows added to the database. What is the problem here? Why can't he update these rows?

[1330 byte] By [TheNetStriker] at [2007-12-16]