From what I understand, the order of the keys in an index is important, since mongo can use any index that has additional keys after the keys required for a query. Thus index(A,B,C) is different from index(C,B,A)

1. ensureIndex() takes a BasicDBObject as an argument.
2. The code:

public static String genIndexName( DBObject keys ){
String name = "";
for ( String s : keys.keySet() )

{ if ( name.length() > 0 ) name += "_"; name += s + "_"; Object val = keys.get( s ); if ( val instanceof Number || val instanceof String ) name += val.toString().replace( ' ' , '_' ); }

return name;
}

3. Generates a name for a DBObject. When there are multiple keys, "key1": 1, "key2":2, keyset() will return the data in arbitrary order. The name will then be the same for each SET of keys.
4. in ensureIndex():

final String name = options.get( "name" ).toString();

if ( _createIndexes.contains( name ) )
return;

Will fail, if any other index with the same keys exist, regardless of order.

5. Even if that does not fail, and a name is manually supplied, createIndex's comment implies that it uses a set of fields, which will not preserve order /** Forces creation of an index on a set of fields, if one does not already exist.

• @param keys an object with a key set of the fields desired for the index
  */
  public final void createIndex( final DBObject keys )
  throws MongoException { createIndex( keys , defaultOptions( keys ) ); }

6. Mongo behaves correctly when adding indices from the console.