public function expression($value) : self
    {
        $this->requiresCurrentField(__METHOD__);
        $this->expr[$this->currentField] = $this->ensureArray($value);

        return $this;
    }
public function expression($value) : self
    {
        $this->requiresCurrentField(__METHOD__);
        $this->expr[$this->currentField] = $this->ensureArray($value);

        return $this;
    }
public function expression ( $ value ) : self { $ this -> requiresCurrentField ( __METHOD__ ) ; $ this -> expr [ $ this -> currentField ] = $ this -> ensureArray ( $ value ) ; return $ this ; }
Allows any expression to be used as a field value.

@see http://docs.mongodb.org/manual/meta/aggregation-quick-reference/#aggregation-expressions

@param mixed|self $value
Allows any expression to be used as a field value .