public function IndexBy()
{
$this->match(Lexer::T_INDEX);
$this->match(Lexer::T_BY);
$pathExpr = $this->StateFieldPathExpression();
// Add the INDEX BY info to the query component
$this->_queryComponents[$pathExpr->identificationVariable]['map'] = $pathExpr->field;
return new AST\IndexBy($pathExpr);
}