Nested JSON objects/arrays from subqueries

[aleclarson]

db.select(
  t.tweet(tweet => ({
    // Many-to-one relation
    author: db.get(t.user, tweet.author),
    // Many-to-many relation
    hashtags: db.select(t.hashtag).where(hashtag => hashtag.id.in(tweet.hashtags)),
    // One-to-many relation
    likes: db.select(t.like).where(like => like.tweet.eq(tweet.id)),
  }))
)

That query would produce an array of objects with the following shape:

const result = {
  author: { ... },
  hashtags: [ { ... }, ... ],
  likes: [ { ... }, ... ],
}

[aleclarson]

Marking this as low priority, since #14 covers the most poignant use cases.