How should get value an other table by foreign key in hamlet?

Question

I have data Book and Author, and Books has a foreign key AuthorId. I would like to list out books from DB. I need the name of author but book entity just has id of author that why I use getfunction to get Author data, but in Hamlet file I could not get the name of Author, because getfunction return Maybe Author.

Let see my code:

 Book
   isbn  Text
   title Text
   description Textarea Maybe
   author AuthorId
   UniqueBook isbn
 Author
   name Text
   UniqueAuthor name

The get function of request:

getBookListR :: Handler Html
getBookListR = do
        books <- runDB $ selectList [BookTitle !=. ""] []
        defaultLayout $ do
                $(widgetFile "booklistpage")

The booklistpage hamlet file content:

 $if not $ null books
           <table .table>
           $forall Entity bookId book <- books
             <tr>
                <th th rowspan="4">Image
                <td> #{bookTitle book} 
             <tr>
                   <td> 
                      $maybe author <- get (bookAuthor book)
                            #{authorName author}
             <tr>
                 <td> #{bookIsbn book}

               <tr>
                 <td> 
                      $maybe description <- bookDescription book
                            #{description} 

this part of the code has a problem

         <td> 
             $maybe author <- get (bookAuthor book)
                     #{authorName author}

I got this error:

 Handler\BookList.hs:9:19:
    Couldn't match expected type `Author'
            with actual type `Maybe Author'
    In the first argument of `authorName', namely `author_afBtA'
    In the first argument of `toHtml', namely `authorName author_afBtA'
    In the first argument of `asWidgetT . toWidget', namely
     `toHtml (authorName author_afBtA)'

I thought that $maybe will help me, but perhaps I misunderstand the concept. I would like to understand why this code does not work, and what the solution for these kind of cases, when we have only the key when we iterate in hamlet file.

Answer 1

I actually recently added a chapter on SQL joins to the Yesod book, that covers almost exactly this case.