Signaling relationship changes

How do you implement one-to-one, one-to-many, and many-to-many relationships? How do you signal that a new entity in a collection was created? How do you mark it deleted?

For example, when we do post.addUpvote(vote) or post.removeUpvote(vote), we're adding or removing an entity (PostUpvote), where a Post can have many PostUpvotes, from Post.

To solve this particular problem, I rolled myself a WatchedList<T> base class that keeps track of the initial, new, and deleted items in a collection of entities, like the collection of PostUpvotes

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