Computed fields

The built-in computed field is a count over a collection relation:

const Post = entity('Post', posts, {
  relations: {
    comments: many(Comment, {
      foreignKey: comments.postId,
      localKey: posts.id,
    }),
  },
  computed: {
    commentCount: { relation: 'comments' },
  },
})

commentCount becomes a Remote number field. The Source runs a grouped count(*), including the relation's static where and principal policy. The count is the total relation count, not the current page count.

Other aggregate shapes are intentionally not a generic compiler yet.