If the field does not exist, it will be set to an array containing the
unique value(s) in the argument. If the field is not an array, the query
will yield an error.
Multiple values may be specified by provided an Expr object and using
{@link Expr::each()}.
또한 보기:
Expr::addToSet()
또한 보기:
http://docs.mongodb.org/manual/reference/operator/addToSet/
또한 보기:
http://docs.mongodb.org/manual/reference/operator/each/