Unclear how to use simple newtype wrappers in beam table columns

[codygman]

Related to #262.

I think it expands on that because I try something beyond what the recommendations there cover and use just_ with my that custom column type:

guard_' (just_ emails ==?. _userEmail users)

I tailored that from the example on values support.

I hope you don't mind, most of my commentary is inline comments in the Haskell code.

data UserT f
  = UserT { _userId         :: Columnar f (SqlSerial Int)
          , _userEmail   :: Columnar f Email
  }
type User = UserT Identity

textToEmail :: Text.Text -> Maybe Email
textToEmail = undefined

newtype Email = Email Text.Text

instance Beam.HasSqlEqualityCheck Beam.Postgres Email

instance Beam.FromBackendRow Beam.Postgres Email where
  fromBackendRow = do
    val <- Beam.fromBackendRow
    case textToEmail val of
      Right email -> pure email
      Left str -> fail $ "invalid email: " <> str

-- that works until I want to do something like
-- NOTE assuming this is like
-- select * from users where user.email in ('email1@email.com', ...)
getUsersByEmail :: [Email] -> IO [User]
getUsersByEmail listOfEmails = runSelectReturningList @Beam.Postgres @Beam.Pg @User . select $ do
      users <- all_ (ViewingFact._metricsViewingFact ViewingFact.metricsDb)
      emails <- values_ listOfEmails
      guard_' (just_ emails ==?. _userEmail users)
      pure users

If time allows I'll make a repo of this example for others who run into this problem and if necessary for us to debug with.

[codygman]

Maybe beam-newtype-field addresses this, but I'm thinking it will only address the FromBackendRowinstance and I need the Justable instance anyway.

[codygman]

I tried to debug this and validate my thought that the above produces the query I even want, and I got stuck at UUID equality not being defined for the MockSqlBackend:

dumpSqlSelect $ do users <- all_ (_userUuid userDB); val <- values_ []; guard_' (val ==?. _userUuid users); pure users
<interactive>:68:119: error:
    * No instance for (HasSqlEqualityCheck
                         (Database.Beam.Backend.SQL.MockSqlBackend
                            Database.Beam.Backend.SQL.Builder.SqlSyntaxBuilder)
                         uuid-types-1.0.3:Data.UUID.Types.Internal.UUID)
        arising from a use of `==?.'
    * In the first argument of guard_', namely
        `(val ==?. _userUuid users)'
      In a stmt of a 'do' block:
        guard_' (val ==?. _userUuid users)
      In the second argument of `($)', namely
        `do users <- all_
                       (_metricsEmail metricsDb)
            val <- values_ []
            guard_' (val ==?. _userUuid users)
            pure users'

So you either need to be able to define the very general version or define a lot of instances starting like:

deriving newtype instance (Beam.IsSql92Syntax sql, Beam.HasSqlEqualityCheck (Beam.MockSqlBackend sql) UUID.UUID)

Or am I using it wrong?

[tathougies]

@codygman FWIW, the mock sql backend is really only for debugging. Your example will likely work if used on an actual backend. Unfortunately, writing out the sql for a particular backend is not always trivial. Postgres for example requires us to have an active connection to be able to determine the appropriate syntax (long story...). Have you tried simply using the query? There are backend-specific debug functions to allow you to show the queries being run as they are running (they pass a string representation to an IO action).

[srid]

This works for me:

newtype BranchName = BranchName {unBranchName :: Text}
  deriving stock (Generic)
  deriving newtype
    ( Show
    , Eq
    , Ord
    , IsString
    , ToJSON
    , ToString
    , FromBackendRow Sqlite
    , HasSqlValueSyntax SqliteValueSyntax
    )