Hydrate related nodes before persisting a relationship

@gerrit.meier, this might warrant a new topic but I'll give it a shot.

What would be the best way, in your opinion, to hydrate related entities before calling save, to prevent removing information from those entities.

For example, if a client provides an object with a list of IDs as related entities. What's the most efficient way of creating those relationships without affecting the original nodes if they already exist?