Sort rows in data.table in decreasing order on string key `order(-x,v)` gives error on data.table 1.9.4 or earlier

Let's say I have the following data.table in R:

  library(data.table)
  DT = data.table(x=rep(c("b","a","c"),each=3), y=c(1,3,6), v=1:9)

I want to order it by two columns (say columns x and v). I used this:

 DT[order(x,v)] # sorts first by x then by v (both in ascending order)

But now, I want to sort it by x (in decreasing order) and have the following code:

  DT[order(-x)] #Error in -x : invalid argument to unary operator

Therefore, I think this error is due to the fact that class(DT$x)=character. Could you give me any suggestion in order to solve this issue?

I know I can use DT[order(x,decreasing=TRUE)], but I want to know the syntax to sort by several columns using both ways (some decreasing, some increasing) at the same time.

Note that if you use DT[order(-y,v)] the result is ok, but if you use DT[order(-x,v)] there is an error. So, my question is: how to solve this error?

Update

data.table v1.9.6+ now supports OP's original attempt and the following answer is no longer necessary.

---

You can use DT[order(-rank(x), y)].

   x y v
1: c 1 7
2: c 3 8
3: c 6 9
4: b 1 1
5: b 3 2
6: b 6 3
7: a 1 4
8: a 3 5
9: a 6 6

You can only use - on the numeric entries, so you can use decreasing and negate the ones you want in increasing order:

DT[order(x,-v,decreasing=TRUE),]
      x y v
 [1,] c 1 7
 [2,] c 3 8
 [3,] c 6 9
 [4,] b 1 1
 [5,] b 3 2
 [6,] b 6 3
 [7,] a 1 4
 [8,] a 3 5
 [9,] a 6 6

DT[order(-x)] works as expected. I have data.table version 1.9.4. Maybe this was fixed in a recent version.
Also, I suggest the setorder(DT, -x) syntax in keeping with the set* commands like setnames, setkey