The server doesn't know what groups are on the client beforehand. I would have to change the design so that before the merge begins, you have to connect the devices, then the client would have to send over its database, and then the server could query the database to get all the groups, and then you would be able to configure the sync behavior. It would add a lot more complexity to have to track the merge behavior per collection. Honestly, I think that's all overkill though for what most people want to do. Most people just want to merge over one or more updated setlists/collections/etc. I'd rather keep it as simple as possible and just let a user pick the groups they want to merge, and then I calculate all of the songs used by both devices that are a part of the selected groups, and limit the merge to those. If you want complete control over how the merge proceeds (the kind of level of control your proposed dialog would provide), you can achieve basically the same thing by changing the merge behavior to "Prompt user for decision", and then just decide exactly what happens for each song and group.
Mike
Mike