How can I order a Maybe column?

[DougBurke]

With (my actual data is a little more complicated than this, and the column isn't an integer, but is a also an instance of DBOrd)

data NotInteresting f = NotInteresting { ni :: Column f (Maybe Int64) }
  deriving stock (Generic)
  deriving anyclass (Rel8able)

notInterestingSchema :: TableSchema (NotInteresting Name)
notInterestingSchema = TableSchema {
  name = "ni"
  , columns = NotInteresting { ni = "nini" }
  }

getNI = orderBy (ni >$< desc) $ do
  row <- each notInterestingSchema
  pure row

I get

    • No instance for ‘DBOrd (Maybe Int64)’
        arising from a use of ‘desc’
    • In the second argument of ‘(>$<)’, namely ‘desc’
      In the first argument of ‘orderBy’, namely ‘(ni >$< desc)’
      In the first argument of ‘($)’, namely ‘orderBy (ni >$< desc)’

(this should be with rel8 1.6.0.0).

Is there a way to add an instance for something like

instance DBOrd a => DBOrd (Maybe a)

although this probably interacts with #310 and the choice of how to handle nulls, or some other way to write a query that lets be order on a maybe column?

[ocharles]

You need https://hackage.haskell.org/package/rel8-1.6.0.0/docs/Rel8.html#v:nullsFirst (or nullsLast). Is that what you're looking for?

[DougBurke]

getNI = orderBy (ni >$< nullsLast desc) $ do
  row <- each notInterestingSchema
  pure row

compiles, so that must be right ....