instance PersistField UUID where
toPersistValue = PersistLiteralEscaped . uuidToBytes
fromPersistValue (PersistLiteralEscaped bytes)
| Just uuid <- fromAsciiBytes bytes = Right uuid
fromPersistValue other =
Left $ "Failed to make UUID from: " <> tshow other
This fails to round-trip if you do a select $ pure $ val uuid - it fails, because we were given a PersistText instead of a PersistLiteralEscaped.
The logic, as far as I can tell, is something like:
We splice in val uuid, which turns into ( "?", toPersistValue uuid) to ("?", PersistLiteralEscaped uuid)
After parameter substitution, postgres sees a '1234-2345-3456-4567' looking thing, which looks a lot like a text value. So that's the type it infers.
postgresql-simple asks Postgres what the type of the column is, and says text.
So persistent wraps the text in a PersistText constructor
And then esqueleto tries to parse a UUID from it, which fails, since the PersistText constructor isn't specified above.
One potential fix is to provide a type annotation to val. Something like,
val x =
( mconcat ["(? :: ", showType (sqlType (Proxy :: Proxy a)), ")"]
, toPersistValue x
)
Another fix is on persistent side, where we encourage folks to use helpers instead of the PersistValue constructors directly. Eg, withBytes :: PersistValue -> (ByteString -> Either Text a) -> Either Text a, similar to the withObject etc functions in aeson. These would, behind the scenes, translate a PersistText txt -> f (encodeUtf8 txt).
We have this instance at work:
This fails to round-trip if you do a
select $ pure $ val uuid- it fails, because we were given aPersistTextinstead of aPersistLiteralEscaped.The logic, as far as I can tell, is something like:
val uuid, which turns into( "?", toPersistValue uuid)to("?", PersistLiteralEscaped uuid)postgressees a'1234-2345-3456-4567'looking thing, which looks a lot like atextvalue. So that's the type it infers.postgresql-simpleasks Postgres what the type of the column is, and saystext.persistentwraps thetextin aPersistTextconstructoresqueletotries to parse aUUIDfrom it, which fails, since thePersistTextconstructor isn't specified above.One potential fix is to provide a type annotation to
val. Something like,Another fix is on
persistentside, where we encourage folks to use helpers instead of thePersistValueconstructors directly. Eg,withBytes :: PersistValue -> (ByteString -> Either Text a) -> Either Text a, similar to thewithObjectetc functions inaeson. These would, behind the scenes, translate aPersistText txt -> f (encodeUtf8 txt).