diff --git a/Database/Persist/MongoDB.hs b/Database/Persist/MongoDB.hs
--- a/Database/Persist/MongoDB.hs
+++ b/Database/Persist/MongoDB.hs
@@ -374,7 +374,7 @@
                   (Multiply, _) -> error "expected PersistInt64 or PersistDouble for a subtraction"
                   -- Obviously this could be supported for floats by multiplying with 1/x
                   (Divide, _)   -> throw $ PersistMongoDBUnsupported "divide not supported"
-updateToMongoField (BackendUpdate _) = error "no backend updates implemented yet"
+updateToMongoField (BackendUpdate {}) = error "no backend updates implemented yet"
 
 
 -- | convert a unique key into a MongoDB document
@@ -588,18 +588,19 @@
             -}
    
 
-_id :: T.Text
-_id = "_id"
+-- | It would make more sense to call this _id, but GHC treats leading underscore in special ways
+id_ :: T.Text
+id_ = "_id"
 
 -- _id is always the primary key in MongoDB
 -- but _id can contain any unique value
 keyToMongoDoc :: (PersistEntity record, PersistEntityBackend record ~ DB.MongoContext)
                   => Key record -> DB.Document
 keyToMongoDoc k = case entityPrimary $ entityDefFromKey k of
-    Nothing   -> zipToDoc [DBName _id] values
-    Just pdef -> [_id DB.=: zipToDoc (primaryNames pdef)  values]
+    Nothing   -> zipToDoc [DBName id_] values
+    Just pdef -> [id_ DB.=: zipToDoc (primaryNames pdef)  values]
   where
-    primaryNames = map fieldDB . primaryFields
+    primaryNames = map fieldDB . compositeFields
     values = keyToValues k
 
 entityDefFromKey :: PersistEntity record => Key record -> EntityDef
@@ -659,7 +660,7 @@
         context <- ask
         let make = do
                 cursor <- liftIO $ flip runReaderT context $ DB.find $ (makeQuery filts opts) {
-                    DB.project = [_id DB.=: (1 :: Int)]
+                    DB.project = [id_ DB.=: (1 :: Int)]
                   }
                 pull context cursor
         return $ return make
@@ -807,8 +808,10 @@
       Right vs -> DB.val $ map toPersistValue vs
 
 fieldName ::  forall record typ.  (PersistEntity record) => EntityField record typ -> DB.Label
-fieldName = idfix . unDBName . fieldDB . persistFieldDef
-  where idfix f = if f == "id" then _id else f
+fieldName f | fieldHaskell fd == HaskellName "Id" = id_
+            | otherwise = unDBName $ fieldDB $ fd
+  where
+    fd = persistFieldDef f
 
 
 docToEntityEither :: forall record. (PersistEntity record) => DB.Document -> Either T.Text (Entity record)
@@ -837,7 +840,7 @@
 eitherFromPersistValues entDef doc =
     let castDoc = assocListFromDoc doc
         -- normally _id is the first field
-        mKey = lookup _id castDoc
+        mKey = lookup id_ castDoc
     in case mKey of
          Nothing -> Left "could not find _id field"
          Just kpv -> fromPersistValues (map snd $ orderPersistValues (toEmbedEntityDef entDef) castDoc)
diff --git a/persistent-mongoDB.cabal b/persistent-mongoDB.cabal
--- a/persistent-mongoDB.cabal
+++ b/persistent-mongoDB.cabal
@@ -1,5 +1,5 @@
 name:            persistent-mongoDB
-version:         2.0.2
+version:         2.0.3
 license:         MIT
 license-file:    LICENSE
 author:          Greg Weber <greg@gregweber.info>
@@ -19,7 +19,7 @@
 
 library
     build-depends:   base               >= 4 && < 5
-                   , persistent         >= 2.0.2   && < 2.1
+                   , persistent         >= 2.0.3   && < 2.1
                    , text               >= 0.8
                    , transformers       >= 0.2.1
                    , containers         >= 0.2