Attaching references after the build

[seancdavis]

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.

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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[].

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Having both together would be ideal.