Attaching references after the build

The simple version of this is that I have two document types and I want to cache some static field on a document _after_ the build has completed.

For example, let's say I have tags and posts, and I want a tag object to have a `postCount` attribute that caches the number of posts to which the tag was applied.

---

The more complex example is to be able to "hydrate" references without needing to write my own transformer function.

In the example above, posts would have a `tags` field reference. But I'd love if what were returned was of type `Tag[]` rather than `string[]`.

---

Having both together would be ideal.