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

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

        return $this;
    }
public function addToSet ( $ field , $ expression ) { $ this -> stage [ $ field ] [ '$addToSet' ] = Expression :: normalize ( $ expression ) ; return $ this ; }
Add to set accumulator

Returns an array of all unique values that results from applying an
expression to each document in a group of documents that share the
same group by key. Order of the elements in the output
array is unspecified.

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

@param string $field
@param literal|callable|\Sokil\Mongo\Pipeline\Expression $expression Expression
@return \Sokil\Mongo\Pipeline\GroupStage
Add to set accumulator