We have the concept of property-level change tracking, which means that in a given modified entity only some of the properties may be marked as modified. If possible we should only update the values for properties that are marked as modified. I'm not sure about how to handle fields that are not mapped--we may be able to use shadow state, but this might not work if we don't know when building the model which fields will be there. I'm find with us not handling either of these things for now, but I think we should have a TODO or an issue to track them (if we don't already have one, which I didn't check. :-))