Revert "Revert "custom primary keys (not composite, yet) supported on projections""

This reverts commit 761ef03b03.

src/Database/Esqueleto/Internal/Sql.hs

@@ -8,6 +8,7 @@
            , OverloadedStrings
            , UndecidableInstances
            , ScopedTypeVariables
+           , InstanceSigs
  #-}
 -- | This is an internal module, anything exported by this module
 -- may change without a major version bump.  Please use only
@@ -60,7 +61,8 @@ import Data.Monoid (Monoid(..), (<>))
 import Data.Proxy (Proxy(..))
 import Database.Esqueleto.Internal.PersistentImport
 import Database.Persist.Sql.Util (
-    entityColumnNames, entityColumnCount, parseEntityValues)
+    entityColumnNames, entityColumnCount, parseEntityValues, isIdField
+  , hasCompositeKey)
 import qualified Control.Monad.Trans.State as S
 import qualified Control.Monad.Trans.Writer as W
 import qualified Data.Conduit as C
@@ -336,8 +338,16 @@ instance Esqueleto SqlQuery SqlExpr SqlBackend where
   sub_select         = sub SELECT
   sub_selectDistinct = sub SELECT_DISTINCT
 
-  EEntity ident ^. field =
-    ERaw Never $ \info -> (useIdent info ident <> ("." <> fieldName info field), [])
+  (^.) :: forall val typ. (PersistEntity val, PersistField typ) =>
+          SqlExpr (Entity val) -> EntityField val typ -> SqlExpr (Value typ)
+  EEntity ident ^. field
+    | isIdField field && hasCompositeKey ed
+    = ERaw Parens $
+      \info@(conn,_) -> (uncommas (map (\a -> useIdent info ident <> "." <> TLB.fromText (connEscapeName conn (fieldDB a))) (compositeFields pdef)), [])
+    | otherwise = ERaw Never $ \info -> (useIdent info ident <> ("." <> fieldName info field), [])
+    where
+      ed = entityDef $ getEntityVal $ (Proxy :: Proxy (SqlExpr (Entity val)))
+      Just pdef = entityPrimary ed
 
   EMaybe r ?. field = maybelize (r ^. field)
     where