Allow ON clauses on sub-JOINs.

src/Database/Esqueleto/Internal/Sql.hs

@@ -10,7 +10,7 @@ module Database.Esqueleto.Internal.Sql
 import Control.Applicative (Applicative(..), (<$>))
 import Control.Arrow ((***), first)
 import Control.Exception (throw, throwIO)
-import Control.Monad (ap)
+import Control.Monad (ap, MonadPlus(..))
 import Control.Monad.IO.Class (MonadIO(..))
 import Control.Monad.Logger (MonadLogger)
 import Control.Monad.Trans.Resource (MonadResourceBase)
@@ -79,10 +79,18 @@ collectOnClauses = go []
     go acc (f:fs)                 = go (f:acc) fs
     go acc []                     = return $ reverse acc
 
-    findMatching (FromJoin l k r Nothing : acc) expr =
-      return (FromJoin l k r (Just expr) : acc)
-    findMatching (f : acc) expr = (f:) <$> findMatching acc expr
-    findMatching []        expr = Left expr
+    findMatching (f : acc) expr =
+      case tryMatch expr f of
+        Just f' -> return (f' : acc)
+        Nothing -> (f:) <$> findMatching acc expr
+    findMatching [] expr = Left expr
+
+    tryMatch expr (FromJoin l k r Nothing) =
+      return (FromJoin l k r (Just expr))
+    tryMatch expr (FromJoin l k r j@(Just _)) =
+      ((\r' -> FromJoin l k r' j) <$> tryMatch expr r) `mplus`
+      ((\l' -> FromJoin l' k r j) <$> tryMatch expr l)
+    tryMatch _ _ = mzero
 
 
 -- | A complete @WHERE@ clause.