public function push($field, $expression)
    {
        $this->stage[$field]['$push'] = Expression::normalize($expression);

        return $this;
    }
public function push($field, $expression)
    {
        $this->stage[$field]['$push'] = Expression::normalize($expression);

        return $this;
    }
public function push ( $ field , $ expression ) { $ this -> stage [ $ field ] [ '$push' ] = Expression :: normalize ( $ expression ) ; return $ this ; }
Returns an array of all values that result from applying an expression
to each document in a group of documents that share the same group by key.

@link http://docs.mongodb.org/manual/reference/operator/aggregation/push

@param string $field
@param literal|callable|\Sokil\Mongo\Pipeline\Expression $expression Expression
@return \Sokil\Mongo\Pipeline\GroupStage
Returns an array of all values that result from applying an expression to each document in a group of documents that share the same group by key .