Add find using the runCommand operation

Database/MongoDB/Query.hs

@@ -29,7 +29,7 @@ module Database.MongoDB.Query (
     -- ** Query
     Query(..), QueryOption(NoCursorTimeout, TailableCursor, AwaitData, Partial),
     Projector, Limit, Order, BatchSize,
-    explain, find, findOne, fetch,
+    explain, find, findCommand, findOne, fetch,
     findAndModify, findAndModifyOpts, FindAndModifyOpts(..), defFamUpdateOpts,
     count, distinct,
     -- *** Cursor
@@ -77,8 +77,8 @@ import Control.Monad.Reader (MonadReader, ReaderT, runReaderT, ask, asks, local)
 import Control.Monad.Trans (MonadIO, liftIO)
 import Data.Binary.Put (runPut)
 import Data.Bson (Document, Field(..), Label, Val, Value(String, Doc, Bool),
-                  Javascript, at, valueAt, lookup, look, genObjectId, (=:),
-                  (=?), (!?), Val(..), ObjectId, Value(..))
+                  Javascript, at, valueAt, lookup, look, genObjectId, merge,
+                  (=:), (=?), (!?), Val(..), ObjectId, Value(..))
 import Data.Bson.Binary (putDocument)
 import Data.Text (Text)
 import qualified Data.Text as T
@@ -130,6 +130,7 @@ data Failure =
     | WriteConcernFailure Int String  -- ^ Write concern error. It's reported only by insert, update, delete commands. Not by wire protocol.
     | DocNotFound Selection  -- ^ 'fetch' found no document matching selection
     | AggregateFailure String -- ^ 'aggregate' returned an error
+    | FindFailure String -- ^ 'find' returned an error
     | CompoundFailure [Failure] -- ^ When we need to aggregate several failures and report them.
     | ProtocolFailure Int String -- ^ The structure of the returned documents doesn't match what we expected
     deriving (Show, Eq, Typeable)
@@ -1032,6 +1033,33 @@ find q@Query{selection, batchSize} = do
     dBatch <- liftIO $ request pipe [] qr
     newCursor db (coll selection) batchSize dBatch
 
+findCommand :: (MonadIO m, MonadFail m) => Query -> Action m Cursor
+-- ^ Fetch documents satisfying query using the command "find"
+findCommand Query{..} = do
+    let aColl = coll selection
+    response <- runCommand $
+        [ "find"        =: aColl
+        , "filter"      =: selector selection
+        , "sort"        =: sort
+        , "projection"  =: project
+        , "hint"        =: hint
+        , "skip"        =: toInt32 skip
+        ]
+        ++ mconcat -- optional fields
+             [ "batchSize" =? toMaybe (/= 0) toInt32 batchSize
+             , "limit"     =? toMaybe (/= 0) toInt32 limit
+             ]
+
+    getCursorFromResponse aColl response FindFailure
+    where
+      toInt32 :: Integral a => a -> Int32
+      toInt32 = fromIntegral
+
+      toMaybe :: (a -> Bool) -> (a -> b) -> a -> Maybe b
+      toMaybe predicate f a
+        | predicate a = Just (f a)
+        | otherwise   = Nothing
+
 findOne :: (MonadIO m) => Query -> Action m (Maybe Document)
 -- ^ Fetch first document satisfying query or @Nothing@ if none satisfy it
 findOne q = do
@@ -1319,14 +1347,22 @@ aggregateCursor :: (MonadIO m, MonadFail m) => Collection -> Pipeline -> Aggrega
 -- ^ Runs an aggregate and unpacks the result. See <http://docs.mongodb.org/manual/core/aggregation/> for details.
 aggregateCursor aColl agg _ = do
     response <- runCommand ["aggregate" =: aColl, "pipeline" =: agg, "cursor" =: ([] :: Document)]
-    case true1 "ok" response of
-        True  -> do
-          cursor :: Document <- lookup "cursor" response
-          firstBatch :: [Document] <- lookup "firstBatch" cursor
-          cursorId :: Int64 <- lookup "id" cursor
-          db <- thisDatabase
-          newCursor db aColl 0 $ return $ Batch Nothing cursorId  firstBatch
-        False -> liftIO $ throwIO $ AggregateFailure $ at "errmsg" response
+    getCursorFromResponse aColl response AggregateFailure
+
+getCursorFromResponse
+  :: (MonadIO m, MonadFail m)
+  => Collection
+  -> Document
+  -> (String -> Failure)
+  -> Action m Cursor
+getCursorFromResponse aColl response err
+  | true1 "ok" response = do
+      cursor     <- lookup "cursor" response
+      firstBatch <- lookup "firstBatch" cursor
+      cursorId   <- lookup "id" cursor
+      db         <- thisDatabase
+      newCursor db aColl 0 (return $ Batch Nothing cursorId firstBatch)
+  | otherwise = liftIO . throwIO . err $ at "errmsg" response
 
 -- ** Group