Persistent upsert does not compile

Question

I'm trying to do something in Persistent with PostgreSQL that seems like it should be simple: Given a list of Attendance records, overwrite any existing rows with same unique key or insert if they don't exist. Here are the types:

share [mkPersist sqlSettings, mkDeleteCascade sqlSettings, mkMigrate "migrateAllEvents"] [persistLowerCase|
Event json sql=events
    description Text
    date UTCTime
    UniqueEvent date
Attendance json
    attending Bool
    eventId EventId
    user UserId
    UniqueAttendance eventId user
|]

I don't have keys, so I can't use repsert , but I think upsert is what I need. The documentation for upsert looks a bit vague to me: If the record exists and I leave the updates blank, I understand that it will retain the entity as it was. But that's not equivalent to the behavior of repsert , as claimed, is it? It also doesn't say what happens when no uniqueness constraint exists.

What I tried was

runDb (mapM_ ups atts)
 where 
  -- ups rec = upsert rec [AttendanceAttending =. True] also doesn't work
  ups rec = upsert rec [AttendanceAttending =. (attendanceAttending rec)]

but this leads to an error that I don't really understand:

• Illegal equational constraint BaseBackend backend ~ SqlBackend
  (Use GADTs or TypeFamilies to permit this)
• When checking the inferred type
    ups :: forall (m :: * -> *) backend.
           (BaseBackend backend ~ SqlBackend, PersistUniqueWrite backend,
            MonadIO m) =>
           Attendance -> ReaderT backend m (Entity Attendance)

Any idea what causes this error? upsertBy is also the same. I have imported Esqueleto in that same file, but hiding ((=.)) , so it should be pure Persistent.

Answer 1

Embarassingly, it really was as simple as adding {-# LANGUAGE TypeFamilies #-} . I hadn't tried that because I misread the error message and none of my other modules needed that extension. Thanks, Fyodor!