I am using MongoDB v1.4 and the mongodb-csharp driver and I try to group on a data store that has more than 10000 keys, so I get this error:

assertion: group() can't handle more than 10000 unique keys

using c# code like this:

Document query = new Document().Append("group",
new Document()
.Append("key", new Document().Append("myfieldname", true))
.Append("$reduce",
      new CodeWScope(
          "function(obj,prev) { prev.count++; }"))
.Append("initial", new Document().Append("count", 0))
.Append("ns", "myitems"));

I read that I should use map/reduce, but I can't figure out how. Can somebody please shed some light on how to use map/reduce?
Or is there any other way to get around this limitation?
Thanks.