Support relations as persisted derived fields

[stephenh]

No description provided.

[stephenh]

@chr1sjf0x I did a spike of this in #416 ; I got maybe 80% of the "recalc + save to the db" mechanism done, but then realized that these will be need to be worked into load hints.

I.e. I made a Author.favoriteBook: Book that is calculated from the Author.books collection.

When someone does em.load(Author, "a:1"), we know "the favorite book is b:1", but we haven't actually loaded the Book entity yet, i.e. just like any other FK. So even though the PersistedDerivedProperty.get method is available:

const a = await em.load(Author, "a:1");
console.log(a.favoriteBook.get);

It prints "b:1". So, unlike primitive PersistedDerivedPropertys, I think the user will have to load hint the favoriteBook:

const a = await em.load(Author, "a:1", "favoriteBook");
console.log(a.favoriteBook.get); // <-- now `Book`

Which is interesting b/c favoriteBook will now essentially have two different loaded states:

1. Is my last-derived-value loaded, i.e. is the value b:1 loaded (which needs to be loaded anytime you want to call .get)
2. Is my recalc-load-hint loaded, i.e. is books loaded so I could re-calc the value (which only needs to be loaded when Joist has detected favoriteBook should be recalced)

So I think we'll probably need some sort of new PersistedAyncRelation that both a) hides the .get until it's loaded, b) is integrated into MarkLoaded similar to ManyToOneReference, c) the implementation is a blend of PersistedAsyncField and ManyToOneReference to balance the dual "value loaded" / "recalc loaded" state.

This all seems doable, but has grown outside of my current/expired timebox. :-) I published #416 with the wip and we can pick it up from there at some point.

[stephenh]

This was fixed in #639 🎉