Hasql nested records and maybe/nullable

README.md

Start postgresql:

docker run --rm --name postgresql -e POSTGRES_PASSWORD=password -p 5432:5432 postgres

Connect to it in separate terminal:

docker exec -it postgresql psql -U postgres

And populate DB:

CREATE TABLE animal (id SERIAL PRIMARY KEY, name TEXT NOT NULL);
CREATE TABLE house (title TEXT NOT NULL, location TEXT NOT NULL, animal INTEGER REFERENCES animal);
INSERT INTO animal (name) VALUES ('dog');

And run our query which results into unexpected result given how line 31 is defined:

$ ./HaSQLNestedRecord.hs 
Left (QueryError "SELECT animal.id, animal.name, house.title, house.location FROM animal LEFT JOIN house ON house.animal = animal.id WHERE animal.id = $1" ["1"] (ResultError (RowError 0 UnexpectedNull)))

HasqlNestedMaybeRecord.hs

#!/usr/bin/env stack
-- stack --nix --nix-packages postgresql --resolver lts-16.23 --install-ghc runghc --package hasql --package text --package bytestring
{-# LANGUAGE OverloadedStrings #-}

import Data.Text
import Data.ByteString
import Data.Int
import qualified Hasql.Decoders as HD
import qualified Hasql.Encoders as HE
import qualified Hasql.Statement as HQ
import qualified Hasql.Connection as HC
import qualified Hasql.Session as HS

data Animal = Animal 
  { id :: Int32
  , name :: Text
  , house :: Maybe House
  } deriving Show

data House = House 
  { title :: Text 
  , location :: Text
  } deriving Show


decoder :: HD.Row Animal
decoder =
  Animal
    <$> fmap fromIntegral ((HD.column . HD.nonNullable) HD.int4)
    <*> (HD.column . HD.nonNullable) HD.text
    <*> fmap Just houseDecoder

houseDecoder :: HD.Row House
houseDecoder =  
  House 
    <$> (HD.column . HD.nonNullable) HD.text
    <*> (HD.column . HD.nonNullable) HD.text

selectOneAnimal :: HQ.Statement Int32 Animal
selectOneAnimal =
  HQ.Statement sql ((HE.param . HE.nonNullable) HE.int4) (HD.singleRow decoder) True
  where
    sql :: ByteString
    sql =
      "SELECT animal.id, animal.name, house.title, house.location \
      \FROM animal \
      \LEFT JOIN house ON house.animal = animal.id \
      \WHERE animal.id = $1"

main :: IO ()
main = do
  eitherConnection <- HC.acquire "postgresql://postgres:password@localhost/postgres"
  case eitherConnection of 
    Left err -> print err
    Right connection -> do
      animal <- HS.run (HS.statement 1 selectOneAnimal) connection
      print animal