Danon5
It seems that there is currently no way to defer entity relation changes without a custom command buffer system. Additionally, there is no way to track relation changes via events or the EventRecorder. As of now, relations are _almost_ useful, but these are some big caveats to have. If these features were in place, relations would be extremely useful for something like collisions. Having a collision enter and exit based on relation add / remove would be an effective way to tackle the problem.