What's the solution to referring to other "objects" in this setup? For example, an Author has a list of Books that they wrote. I see several possible ways to do

What's the solution to referring to other "objects" in this setup? For example, an Author has a list of Books that they wrote. I see several possible ways to do this, but they all seem to have downsides. 1) The author map has a "books" entry that is a list of map containing the book data. But multiple authors might have written the same book, so does the book data get copied there? 2) The author contains a "books" entry that is a list of Book IDs, like foreign keys in a database. But how about immutability? If you want to get the book addressed by the ID, you need to look it up somewhere and are you guaranteed the object you get from the lookup is still the same one as it was earlier? 3) There is a "Author -> Book" map somewhere that you can pass an author and it gives you the list of books they wrote. Not sure about this one.