如何使用外键获取关联数据

Question

I am trying to write a function that look for the profile of a user or create one if it doesn't exists.

I have used getBy and selectFirst to get the profile of a given user, but I get this error:

Couldn't match type 'HandlerFor site0' with 'Key'

I am using the scaffolding site with postgres.

This is my model (user and profile have a one-to-one relationship)

User
    email Text
    password Text Maybe
    verkey Text Maybe
    verified Bool
    UniqueUser email                                                                                                   
    deriving Typeable

Profile 
    name Text                                                                                                          
    userId UserId
    UniqueName name
    UniqueUserId userId
    deriving Typeable

The function is as follow:

getOrCreateProfile :: UserId -> ProfileId
getOrCreateProfile userId = do
    mProfile <- runDB $ getBy $ UniqueUserId userId                                                                    
    case mProfile of
        Just (Entity pid _) -> return pid
        Nothing  -> undefined -- insert profile

The error I am getting is:

    • Couldn't match type ‘HandlerFor site0’ with ‘Key’
      Expected type: Key (Maybe (Entity Profile))
        Actual type: HandlerFor site0 (Maybe (Entity Profile))
    • In a stmt of a 'do' block:
        mProfile <- runDB $ getBy $ UniqueUserId userId
      In the expression:
        do mProfile <- runDB $ getBy $ UniqueUserId userId
           case mProfile of
             Just (Entity pid _) -> pid
             Nothing -> undefined
      In an equation for ‘getOrCreateProfile’:
          getOrCreateProfile userId
            = do mProfile <- runDB $ getBy $ UniqueUserId userId
                 case mProfile of
                   Just (Entity pid _) -> pid
                   Nothing -> undefined
   |
45 |     mProfile <- runDB $ getBy $ UniqueUserId userId
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

What am I doing wrong? What's the proper way to do this query?

Answer 1

If you look at runDB's type signature :

runDB :: YesodDB site a -> HandlerFor site a

you'll see what the problem is.

Yesod requires you to do some rather elaborate stuff to actually use the query's results - it isn't simply a (Maybe result). You can find examples here ; this part in particular:

people <- runDB $ selectList [] [Asc PersonAge]
defaultLayout
        [whamlet|
            <ul>
                $forall Entity personid person <- people
                    <li>
                        <a href=@{PersonR personid}>#{personFirstName person}
        |]

Hope this helps.

Answer 2

This is compiling

getOrCreateProfile :: UserId -> Handler ProfileId
getOrCreateProfile userId = runDB $ do
    mProfile <- getBy $ UniqueUserId userId
    case mProfile of
        Just (Entity pid _) -> return pid
        Nothing  -> insert $ Profile getTempProfile userId