Dealing with complex query results in IHP without having to manually specify columns? - ihp

Question

The "Dealing With Complex Query Results" section of https://ihp.digitallyinduced.com/Guide/database.html shows how you can "include extra data" when querying for a database type, by creating a type like this:
data PostWithCommentsCount = PostWithCommentsCount
{ id :: Id Post
, title :: Text
, commentsCount :: Int
}
deriving (Eq, Show)
This works well, but it has the disadvantage that you need to manually specify all the columns of posts to include in the new PostWithCommentsCount type:
instance FromRow PostWithCommentsCount where
fromRow =
PostWithCommentsCount
<$> field
<*> field
<*> field
fetchPostsWithCommentsCount :: (?modelContext :: ModelContext) => IO [PostWithCommentsCount]
fetchPostsWithCommentsCount = do
trackTableRead "posts" -- This is needed when using auto refresh, so auto refresh knows that your action is accessing the posts table
sqlQuery "SELECT posts.id, posts.title, (SELECT COUNT(*) FROM comments WHERE comments.post_id = posts.id) AS comments_count FROM posts" ()
This is tedious to maintain over time, if changes in the posts table means you also have to change this manual query. I think it could be better if the type looked like this:
data PostWithCommentsCount = PostWithCommentsCount
{ post :: Post
, commentsCount :: Int
}
deriving (Eq, Show)
So that I wouldn't have to manually specify all the columns of posts that I'm interested in - I would just get all the whole Post. Is there a way to accomplish this currently?

Answer 1

Yes this is possible if you use the following FromRow instance:
instance FromRow PostWithCommentsCount where
fromRow = do
post <- fromRow
commentsCount <- field
pure PostWithCommentsCount { post, commentsCount }
You can also write this with the operators above like this:
instance FromRow PostWithCommentsCount where
fromRow = PostWithCommentsCount
<$> fromRow
<*> field