How to connect a Snap app to Heroku Postgres

hello-snap.diff

diff --git a/hello-snap.cabal b/hello-snap.cabal
index c759d41..af67b1a 100644
--- a/hello-snap.cabal
+++ b/hello-snap.cabal
@@ -31,6 +31,7 @@ Executable hello-snap
     snap-core                 >= 0.9     && < 0.11,
     snap-server               >= 0.9     && < 0.11,
     snap-loader-static        >= 0.9     && < 0.10,
+    snaplet-postgresql-simple >= 0.5     && < 0.6,
     text                      >= 0.11    && < 1.2,
     time                      >= 1.1     && < 1.5,
     xmlhtml                   >= 0.1
diff --git a/src/Application.hs b/src/Application.hs
index 8378832..43d27e7 100644
--- a/src/Application.hs
+++ b/src/Application.hs
@@ -11,11 +11,13 @@ import Snap.Snaplet
 import Snap.Snaplet.Heist
 import Snap.Snaplet.Auth
 import Snap.Snaplet.Session
+import Snap.Snaplet.PostgresqlSimple
 
 ------------------------------------------------------------------------------
 data App = App
     { _heist :: Snaplet (Heist App)
     , _sess :: Snaplet SessionManager
+    , _db :: Snaplet Postgres
     , _auth :: Snaplet (AuthManager App)
     }
 
diff --git a/src/Site.hs b/src/Site.hs
index cefdc45..19b158d 100644
--- a/src/Site.hs
+++ b/src/Site.hs
@@ -11,14 +11,16 @@ module Site
 ------------------------------------------------------------------------------
 import           Control.Applicative
 import           Data.ByteString (ByteString)
+import qualified Data.ByteString.Char8 as BS
 import qualified Data.Text as T
-import           Snap.Core
-import           Snap.Snaplet
+import           Snap
 import           Snap.Snaplet.Auth
-import           Snap.Snaplet.Auth.Backends.JsonFile
+import           Snap.Snaplet.Auth.Backends.PostgresqlSimple
 import           Snap.Snaplet.Heist
+import           Snap.Snaplet.PostgresqlSimple
 import           Snap.Snaplet.Session.Backends.CookieSession
 import           Snap.Util.FileServe
+import           System.Environment
 import           Heist
 import qualified Heist.Interpreted as I
 ------------------------------------------------------------------------------
@@ -77,12 +79,12 @@ app = makeSnaplet "app" "An snaplet example application." Nothing $ do
     s <- nestSnaplet "sess" sess $
            initCookieSessionManager "site_key.txt" "sess" (Just 3600)
 
-    -- NOTE: We're using initJsonFileAuthManager here because it's easy and
-    -- doesn't require any kind of database server to run.  In practice,
-    -- you'll probably want to change this to a more robust auth backend.
+    connStr <- BS.pack <$> liftIO (getEnv "DATABASE_URL")
+    d <- nestSnaplet "db" db $
+           pgsInit' (pgsDefaultConfig connStr)
     a <- nestSnaplet "auth" auth $
-           initJsonFileAuthManager defAuthSettings sess "users.json"
+           initPostgresAuth sess d
     addRoutes routes
     addAuthSplices h auth
-    return $ App h s a
+    return $ App h s d a