getTree
  :: MonadIO m
  => PersistEntity ent
  => PersistEntityBackend ent ~ backend
  => Key ent
  -> ReaderT SqlBackend m (Maybe (Tree (Entity ent)))
getTree keyId = do
  let entDef :: EntityDef
      entDef = entityDef (Nothing :: Maybe ent)
  _


---

        • Could not deduce (PersistEntity record0)
            arising from a use of ‘entityDef’
          from the context: (MonadIO m,
                             PersistEntity ent,
                             PersistEntityBackend ent ~ backend)
            bound by the type signature for:
                       getTree :: (MonadIO m, PersistEntity ent,
                                   PersistEntityBackend ent ~ backend) =>
                                  Key ent -> ReaderT SqlBackend m (Maybe (Tree (Entity ent)))
            at foo.hs (114,1)-(120,53)
          The type variable ‘record0’ is ambiguous