Support for multiproject builds with nix

- Closes #28

backend/src/Database.hs

@@ -0,0 +1,32 @@
+{-# Language TypeApplications #-}
+{-# Language DataKinds #-}
+{-# Language FlexibleContexts #-}
+{-# Language ConstraintKinds #-}
+module Database
+  ( DBLike
+  , runDB
+  , query
+  , select
+  , gen
+  , fromRel
+  , fromRels
+  , toRel
+  , transaction
+  , SeldaT )
+  where
+
+import Data.Generics.Product
+import Control.Lens (view)
+import Data.Pool (Pool, withResource)
+import Database.Selda.Backend (SeldaConnection, runSeldaT, SeldaT)
+import Database.Selda (query, select, transaction)
+import Database.Selda.Generic (gen, fromRel, fromRels, toRel)
+import ClassyPrelude
+
+type DBLike r m = (MonadIO m, MonadReader r m, MonadBaseControl IO m, MonadMask m, HasField' "database" r (Pool SeldaConnection))
+
+runDB :: DBLike r m => SeldaT m a -> m a
+runDB q = do
+  pool <- view (field @"database")
+  withResource pool $ \conn ->
+    runSeldaT q conn