instance
    (PersistEntity record, PersistEntityBackend record ~ backend, IsPersistBackend backend) =>
    RawSql (Entity record) where
    rawSqlCols escape _ent = (length sqlFields, [intercalate ", " sqlFields])
        where
          sqlFields = map (((name <> ".") <>) . escape)
              $ map fieldDB
              -- Hacky for a composite key because
              -- it selects the same field multiple times
              $ entityKeyFields entDef ++ entityFields entDef
          name = escape (entityDB entDef)
          entDef = entityDef (Nothing :: Maybe record)

newtype EntityWithPrefix (sym :: Symbol) (record :: Type) = EntityWithPrefix (Entity record)

instance
    (KnownSymbol prefix, PersistEntity record, PersistEntityBackend record ~ backend, IsPersistBackend backend) =>
    RawSql (EntityWithPrefix prefix record) where
    rawSqlCols escape _ent = (length sqlFields, [intercalate ", " sqlFields])
        where
          sqlFields = map (((name <> ".") <>) . escape)
              $ map fieldDB
              -- Hacky for a composite key because
              -- it selects the same field multiple times
              $ entityKeyFields entDef ++ entityFields entDef
          name = symbolVal (Proxy :: Proxy prefix)
          entDef = entityDef (Nothing :: Maybe record)

result :: [(EntityWithPrefix "parent" Relationship, EntityWithPrefix "child" Relationship
  <- rawSql "SELECT ??, ?? FROM relationship AS parent LEFT JOIN relationship AS child ON ..." []

-- module Database.Persist.Sql.Raw
      withStmt' colSubsts params sink = do
            srcRes <- rawQueryRes sql params
            liftIO $ with srcRes (\src -> runConduit $ src .| sink)
          where
            sql = T.concat $ makeSubsts colSubsts $ T.splitOn placeholder stmt
            placeholder = "??"
            makeSubsts (s:ss) (t:ts) = t : s : makeSubsts ss ts
            makeSubsts []     []     = []
            makeSubsts []     ts     = [T.intercalate placeholder ts]
            makeSubsts ss     []     = error (concat err)
                where
                  err = [ "rawsql: there are still ", show (length ss)
                        , "'??' placeholder substitutions to be made "
                        , "but all '??' placeholders have already been "
                        , "consumed.  Please read 'rawSql's documentation "
                        , "on how '??' placeholders work."
                        ]