-- | Update based on a given uniqueness constraint or insert:
    --
    -- * insert the new record if it does not exist;
    -- * update the existing record that matches the given uniqueness contraint.
    upsertBy :: (MonadIO m, PersistRecordBackend record backend)
            => Unique record -- ^ uniqueness constraint to find by
            -> record          -- ^ new record to insert
            -> [Update record]
            -- ^ updates to perform if the record already exists (leaving
            -- this empty is the equivalent of performing a 'repsert' on a
            -- unique key)
            -> ReaderT backend m (Entity record)
            -- ^ the record in the database after the operation
    upsertBy uniqueKey record updates = do
        mExists <- getBy uniqueKey
        k <- case mExists of
            Just (Entity k _) -> do
              when (null updates) (replace k record)
              return k
            Nothing           -> insert record
        Entity k `liftM` updateGet k updates

    -- | Update based on a given uniqueness constraint or insert:
    --
    -- * insert the new record if it does not exist;
    -- * update the existing record that matches the given uniqueness constraint.
    upsertBy
        :: (MonadIO m, PersistRecordBackend record backend)
        => Unique record   -- ^ uniqueness constraint to find by
        -> record          -- ^ new record to insert
        -> [Update record] -- ^ updates to perform if the record already exists (leaving
                           -- this empty is the equivalent of performing a 'repsert' on a
                           -- unique key)
        -> ReaderT backend m (Entity record) -- ^ the record in the database after the operation
    upsertBy uniqueKey record updates = do
        mrecord <- getBy uniqueKey
        maybe (insertEntity record) (`updateGetEntity` updates) mrecord
      where
        updateGetEntity (Entity k _) upds =
            (Entity k) `liftM` (updateGet k upds)`

Just (Entity k _) -> do
  when (null updates) (replace k record)
  return k