JAVA-706: Wrap command in $query field if sending to mongos and read …

…preference is not primary or secondaryPreferred (primary is the default and

secondaryPreferred is already covered by slaveOk bit).

src/main/com/mongodb/DB.java

@@ -269,11 +269,7 @@ public CommandResult command( DBObject cmd , int options, ReadPreference readPre
      */
     public CommandResult command( DBObject cmd , int options, ReadPreference readPrefs, DBEncoder encoder ){
         readPrefs = getCommandReadPreference(cmd, readPrefs);
-
-        // TODO: automate an integration test for this condition
-        if (getMongo().isMongosConnection()) {
-            cmd.put(QueryOpBuilder.READ_PREFERENCE_META_OPERATOR, readPrefs.toDBObject());
-        }
+        cmd = wrapCommand(cmd, readPrefs);
 
         Iterator<DBObject> i =
                 getCollection("$cmd").__find(cmd, new BasicDBObject(), 0, -1, 0, options, readPrefs ,
@@ -288,6 +284,22 @@ public CommandResult command( DBObject cmd , int options, ReadPreference readPre
         return cr;
     }
 
+    // Only append $readPreference meta-operator if connected to a mongos, read preference is not primary
+    // or secondary preferred,
+    // and command is an instance of BasicDBObject.  The last condition is unfortunate, but necessary in case
+    // the encoder is not capable of encoding a BasicDBObject
+    // Due to issues with compatibility between different versions of mongos, also wrap the command in a
+    // $query field, so that the $readPreference is not rejected
+    private DBObject wrapCommand(DBObject cmd, final ReadPreference readPrefs) {
+        if (getMongo().isMongosConnection() &&
+                !(ReadPreference.primary().equals(readPrefs) || ReadPreference.secondaryPreferred().equals(readPrefs)) &&
+                cmd instanceof BasicDBObject) {
+            cmd = new BasicDBObject("$query", cmd)
+                    .append(QueryOpBuilder.READ_PREFERENCE_META_OPERATOR, readPrefs.toDBObject());
+        }
+        return cmd;
+    }
+
     /**
      * Executes a database command.
      * @see <a href="http://mongodb.onconfluence.com/display/DOCS/List+of+Database+Commands">List of Commands</a>