How to do a "SELECT ... IN (SELECT ...)" using Esqueleto?

Question

Considering the following two models and a GET /articles/:slug/comments request, I want to retrieve the comments that belong to an article, based on its slug.

Article json sql=articles
    slug        Slug
    title       Text
    description Text
    body        Text
    createdAt   UTCTime default=now()
    updatedAt   UTCTime Maybe default=NULL
    userId      UserId
    UniqueSlug  slug

Comment json sql=comments
    body      Text
    createdAt UTCTime default=now()
    updatedAt UTCTime Maybe default=NULL
    articleId ArticleId
    userId    UserId

Using persistence's rawSql, we can accomplish this as follows

getCommentsForArticle :: Slug -> App (Cmts [Entity Comment])
getCommentsForArticle slug = do
    comments <- runDb $ rawSql stm [toPersistValue slug]
    return (Cmts comments)
        where stm = "SELECT ?? FROM comments \
                    \WHERE article_id IN (\
                        \SELECT id FROM articles WHERE slug = ?)"

However, given that I want to maintain type safety between Haskell and SQL, I want to rewrite this using esqueleto. This is the part I am struggling with. By reading the docs, sub_select seems to be the tool for the job. Here's what I have:

getCommentsForArticle :: Slug -> App (Cmts [Comment])
getCommentsForArticle slug = do
    comments <- E.select $
        E.from $ \cmts -> do
            let subQuery =
                    E.from $ \arts -> do
                        E.where_ $ arts ^. ArticleSlug ==. E.val slug
                        return (arts ^. ArticleId)
            E.where_ $ cmts ^. CommentArticleId ==. E.sub_select subQuery
            return cmts
    return $ Cmts comments

I also noticed the in_ operator, but I can't figure out how to use it nor if it is more appropriate than sub_select.

What am I missing? Is the syntax correct? Thanks.

Answer 1

You'll want something like this

getCommentsForArticle slug = do
  c <- select $ from $ \cmts -> do
         let a = subList_select $ from $ \arts -> do
                 where_ $ arts ^. ArticleSlug ==. val slug
                 return $ arts ^. ArticleId
         where_ $ cmts ^. CommentArticleId `in_` a
         return cmts
  return $ Cmts c