# tasty-groundhog-converters
This library provides a tasty, test harness for groundhog and groundhog-converters.

The key functions are:
``` haskell
roundTripConverter :: Arbitrary a => TestName -> (a -> a -> Bool) -> (Converter a b) -> TestTree
```

and 

``` haskell
goldenSqlConverter :: (PersistEntity b) =>  TestName ->  FilePath -> a ->   (b -> b -> Bool) -> Converter a b  ->   TestTree
```

Which provide tests for serialization (goldenSqlConverter).
and isomorphism (roundTripConverter).

These two together allow a user to quickly add simple testing to a database project using groundhog.

## Usage
From the example:


``` haskell
data Group = Group {
     _people :: Map Integer Person
         }
 deriving (Eq)


-- | A wrapped representation of a Person
data Person = Person { _unPerson :: String}
  deriving (Eq)


-- | To Build up the converter we have to have an arbitrary instance
instance Arbitrary Person where
  arbitrary = Person <$> arbitrary

-- | An Isomorphism between the representation that is pleasent to use in haskell
-- and the one that makes sense to store i.e. 'PersistEntity' 
personMapConverter :: Converter (Map Integer Person) [(Int64,String)]
personMapConverter = mapConverter `composeConverter` fmapConverter (bicomposeConverter integerConverter personConverter)

-- | This converter is embedded in 'personMapConverter'
personConverter :: Converter Person String
personConverter = (_unPerson,Person)


-- | A declaration for group.
mkPersist defaultCodegenConfig [groundhog|
- entity: Group
  constructors:
  - name: Group
    fields:
      - name: _people
        dbName: people
        exprName: MappedIdToPerson
        converter: personMapConverter
- primitive: Person
  converter: personConverter


|]


-- | build a golden test (a single test designed to make sure a representation stays constant over time).
-- The aGroup provided is only used the first time the test is used.  The converter at the top level here
-- is just (id, id) and (==) is used because there is an Eq instance on Group.
exampleGoldenSqlConverter :: TestTree
exampleGoldenSqlConverter = goldenSqlConverter "Test The test GoldenSqlConverter" "TestGolden" aGroup (==) (id,id) 
  where
    aGroup = Group somePeople
    somePeople = (Map.fromList . zip [1 ..] . fmap Person ) ["Margret"]

-- | There are no database hits on a round trip test
-- Converter makes the claim that a Converter is an Isomorphism between the two DataTypes.
-- Round trip tests should verify this.
exampleRoundTripTest :: TestTree
exampleRoundTripTest = roundTripConverter "roundtrip personMapConverter" (==) personMapConverter 

-- | call the example test
tastyTest :: IO ()
tastyTest = defaultMain allTests
 where
    allTests = testGroup "all example groundhog converter tests" [ exampleGoldenSqlConverter
                                                                 , exampleRoundTripTest]
```