bool isUniqueIndexCompatible(const BSONObj& shardKeyPattern,
                                 const BSONObj& uIndexKeyPattern) {

        if ( !uIndexKeyPattern.isEmpty()
             && string( "_id" ) == uIndexKeyPattern.firstElementFieldName() ) {
            return true;
        }

        return shardKeyPattern.isFieldNamePrefixOf( uIndexKeyPattern );
    }